[chore]: Bump github.com/prometheus/client_golang from 1.20.2 to 1.20.3 (#3282)

Bumps [github.com/prometheus/client_golang](https://github.com/prometheus/client_golang) from 1.20.2 to 1.20.3.
- [Release notes](https://github.com/prometheus/client_golang/releases)
- [Changelog](https://github.com/prometheus/client_golang/blob/v1.20.3/CHANGELOG.md)
- [Commits](https://github.com/prometheus/client_golang/compare/v1.20.2...v1.20.3)

---
updated-dependencies:
- dependency-name: github.com/prometheus/client_golang
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

4 files changed, 70 insertions, 30 deletions

diff --git a/go.mod b/go.mod
index 63ee7f190..cd131add8 100644
--- a/go.mod
+++ b/go.mod
@@ -46,7 +46,7 @@ require (
 	github.com/mitchellh/mapstructure v1.5.0
 	github.com/ncruces/go-sqlite3 v0.18.1
 	github.com/oklog/ulid v1.3.1
-	github.com/prometheus/client_golang v1.20.2
+	github.com/prometheus/client_golang v1.20.3
 	github.com/spf13/cobra v1.8.1
 	github.com/spf13/viper v1.19.0
 	github.com/stretchr/testify v1.9.0
diff --git a/go.sum b/go.sum
index fb4d75015..dfc72ad54 100644
--- a/go.sum
+++ b/go.sum
@@ -468,8 +468,8 @@ github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 h1:Jamvg5psRI
 github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/prashantv/gostub v1.1.0 h1:BTyx3RfQjRHnUWaGF9oQos79AlQ5k8WNktv7VGvVH4g=
 github.com/prashantv/gostub v1.1.0/go.mod h1:A5zLQHz7ieHGG7is6LLXLz7I8+3LZzsrV0P1IAHhP5U=
-github.com/prometheus/client_golang v1.20.2 h1:5ctymQzZlyOON1666svgwn3s6IKWgfbjsejTMiXIyjg=
-github.com/prometheus/client_golang v1.20.2/go.mod h1:PIEt8X02hGcP8JWbeHyeZ53Y/jReSnHgO035n//V5WE=
+github.com/prometheus/client_golang v1.20.3 h1:oPksm4K8B+Vt35tUhw6GbSNSgVlVSBH0qELP/7u83l4=
+github.com/prometheus/client_golang v1.20.3/go.mod h1:PIEt8X02hGcP8JWbeHyeZ53Y/jReSnHgO035n//V5WE=
 github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
 github.com/prometheus/client_model v0.6.1 h1:ZKSh/rekM+n3CeS952MLRAdFwIKqeY8b62p8ais2e9E=
 github.com/prometheus/client_model v0.6.1/go.mod h1:OrxVMOVHjw3lKMa8+x6HeMGkHMQyHDk9E3jmP2AmGiY=
diff --git a/vendor/github.com/prometheus/client_golang/prometheus/histogram.go b/vendor/github.com/prometheus/client_golang/prometheus/histogram.go
index 8d35f2d8a..8a4f49a4c 100644
--- a/vendor/github.com/prometheus/client_golang/prometheus/histogram.go
+++ b/vendor/github.com/prometheus/client_golang/prometheus/histogram.go
@@ -1658,7 +1658,10 @@ func addAndResetCounts(hot, cold *histogramCounts) {
 type nativeExemplars struct {
 	sync.Mutex
 
-	ttl       time.Duration
+	// Time-to-live for exemplars, it is set to -1 if exemplars are disabled, that is NativeHistogramMaxExemplars is below 0.
+	// The ttl is used on insertion to remove an exemplar that is older than ttl, if present.
+	ttl time.Duration
+
 	exemplars []*dto.Exemplar
 }
 
@@ -1673,6 +1676,7 @@ func makeNativeExemplars(ttl time.Duration, maxCount int) nativeExemplars {
 
 	if maxCount < 0 {
 		maxCount = 0
+		ttl = -1
 	}
 
 	return nativeExemplars{
@@ -1682,20 +1686,18 @@ func makeNativeExemplars(ttl time.Duration, maxCount int) nativeExemplars {
 }
 
 func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
-	if cap(n.exemplars) == 0 {
+	if n.ttl == -1 {
 		return
 	}
 
 	n.Lock()
 	defer n.Unlock()
 
-	// The index where to insert the new exemplar.
-	var nIdx int = -1
-
 	// When the number of exemplars has not yet exceeded or
 	// is equal to cap(n.exemplars), then
 	// insert the new exemplar directly.
 	if len(n.exemplars) < cap(n.exemplars) {
+		var nIdx int
 		for nIdx = 0; nIdx < len(n.exemplars); nIdx++ {
 			if *e.Value < *n.exemplars[nIdx].Value {
 				break
@@ -1705,17 +1707,46 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 		return
 	}
 
+	if len(n.exemplars) == 1 {
+		// When the number of exemplars is 1, then
+		// replace the existing exemplar with the new exemplar.
+		n.exemplars[0] = e
+		return
+	}
+	// From this point on, the number of exemplars is greater than 1.
+
 	// When the number of exemplars exceeds the limit, remove one exemplar.
 	var (
-		rIdx int // The index where to remove the old exemplar.
-
-		ot    = time.Now() // Oldest timestamp seen.
-		otIdx = -1         // Index of the exemplar with the oldest timestamp.
-
-		md    = -1.0  // Logarithm of the delta of the closest pair of exemplars.
-		mdIdx = -1    // Index of the older exemplar within the closest pair.
-		cLog  float64 // Logarithm of the current exemplar.
-		pLog  float64 // Logarithm of the previous exemplar.
+		ot    = time.Time{} // Oldest timestamp seen. Initial value doesn't matter as we replace it due to otIdx == -1 in the loop.
+		otIdx = -1          // Index of the exemplar with the oldest timestamp.
+
+		md = -1.0 // Logarithm of the delta of the closest pair of exemplars.
+
+		// The insertion point of the new exemplar in the exemplars slice after insertion.
+		// This is calculated purely based on the order of the exemplars by value.
+		// nIdx == len(n.exemplars) means the new exemplar is to be inserted after the end.
+		nIdx = -1
+
+		// rIdx is ultimately the index for the exemplar that we are replacing with the new exemplar.
+		// The aim is to keep a good spread of exemplars by value and not let them bunch up too much.
+		// It is calculated in 3 steps:
+		//   1. First we set rIdx to the index of the older exemplar within the closest pair by value.
+		//      That is the following will be true (on log scale):
+		//      either the exemplar pair on index (rIdx-1, rIdx) or (rIdx, rIdx+1) will have
+		//      the closest values to each other from all pairs.
+		//      For example, suppose the values are distributed like this:
+		//        |-----------x-------------x----------------x----x-----|
+		//                                                   ^--rIdx as this is older.
+		//      Or like this:
+		//        |-----------x-------------x----------------x----x-----|
+		//                                                        ^--rIdx as this is older.
+		//   2. If there is an exemplar that expired, then we simple reset rIdx to that index.
+		//   3. We check if by inserting the new exemplar we would create a closer pair at
+		//      (nIdx-1, nIdx) or (nIdx, nIdx+1) and set rIdx to nIdx-1 or nIdx accordingly to
+		//      keep the spread of exemplars by value; otherwise we keep rIdx as it is.
+		rIdx = -1
+		cLog float64 // Logarithm of the current exemplar.
+		pLog float64 // Logarithm of the previous exemplar.
 	)
 
 	for i, exemplar := range n.exemplars {
@@ -1726,7 +1757,7 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 		}
 
 		// Find the index at which to insert new the exemplar.
-		if *e.Value <= *exemplar.Value && nIdx == -1 {
+		if nIdx == -1 && *e.Value <= *exemplar.Value {
 			nIdx = i
 		}
 
@@ -1738,11 +1769,13 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 		}
 		diff := math.Abs(cLog - pLog)
 		if md == -1 || diff < md {
+			// The closest exemplar pair is at index: i-1, i.
+			// Choose the exemplar with the older timestamp for replacement.
 			md = diff
 			if n.exemplars[i].Timestamp.AsTime().Before(n.exemplars[i-1].Timestamp.AsTime()) {
-				mdIdx = i
+				rIdx = i
 			} else {
-				mdIdx = i - 1
+				rIdx = i - 1
 			}
 		}
 
@@ -1753,8 +1786,12 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 	if nIdx == -1 {
 		nIdx = len(n.exemplars)
 	}
+	// Here, we have the following relationships:
+	// n.exemplars[nIdx-1].Value < e.Value (if nIdx > 0)
+	// e.Value <= n.exemplars[nIdx].Value (if nIdx < len(n.exemplars))
 
 	if otIdx != -1 && e.Timestamp.AsTime().Sub(ot) > n.ttl {
+		// If the oldest exemplar has expired, then replace it with the new exemplar.
 		rIdx = otIdx
 	} else {
 		// In the previous for loop, when calculating the closest pair of exemplars,
@@ -1764,23 +1801,26 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 		if nIdx > 0 {
 			diff := math.Abs(elog - math.Log(n.exemplars[nIdx-1].GetValue()))
 			if diff < md {
+				// The value we are about to insert is closer to the previous exemplar at the insertion point than what we calculated before in rIdx.
+				//                                            v--rIdx
+				// |-----------x-n-----------x----------------x----x-----|
+				//     nIdx-1--^ ^--new exemplar value
+				// Do not make the spread worse, replace nIdx-1 and not rIdx.
 				md = diff
-				mdIdx = nIdx
-				if n.exemplars[nIdx-1].Timestamp.AsTime().Before(e.Timestamp.AsTime()) {
-					mdIdx = nIdx - 1
-				}
+				rIdx = nIdx - 1
 			}
 		}
 		if nIdx < len(n.exemplars) {
 			diff := math.Abs(math.Log(n.exemplars[nIdx].GetValue()) - elog)
 			if diff < md {
-				mdIdx = nIdx
-				if n.exemplars[nIdx].Timestamp.AsTime().Before(e.Timestamp.AsTime()) {
-					mdIdx = nIdx
-				}
+				// The value we are about to insert is closer to the next exemplar at the insertion point than what we calculated before in rIdx.
+				//                                            v--rIdx
+				// |-----------x-----------n-x----------------x----x-----|
+				//     new exemplar value--^ ^--nIdx
+				// Do not make the spread worse, replace nIdx-1 and not rIdx.
+				rIdx = nIdx
 			}
 		}
-		rIdx = mdIdx
 	}
 
 	// Adjust the slice according to rIdx and nIdx.
diff --git a/vendor/modules.txt b/vendor/modules.txt
index 3132ca140..5a04b2f5d 100644
--- a/vendor/modules.txt
+++ b/vendor/modules.txt
@@ -561,7 +561,7 @@ github.com/pkg/errors
 # github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2
 ## explicit
 github.com/pmezard/go-difflib/difflib
-# github.com/prometheus/client_golang v1.20.2
+# github.com/prometheus/client_golang v1.20.3
 ## explicit; go 1.20
 github.com/prometheus/client_golang/internal/github.com/golang/gddo/httputil
 github.com/prometheus/client_golang/internal/github.com/golang/gddo/httputil/header