[chore] Simplify the OTEL setup (#4110)

# Description

This simplifies our OTEL setup by:

* Getting rid of some deprecated things.
* Using `autoexport` and letting things get configured by the `OTEL_` environment variables.
* Removing all the unnecessary config options.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4110
Reviewed-by: tobi <kipvandenbos@noreply.codeberg.org>
Co-authored-by: Daenney <daenney@noreply.codeberg.org>
Co-committed-by: Daenney <daenney@noreply.codeberg.org>

3 files changed, 0 insertions, 157 deletions

diff --git a/internal/api/metrics.go b/internal/api/metrics.go
deleted file mode 100644
index add14b0bb..000000000
--- a/internal/api/metrics.go
+++ /dev/null
@@ -1,66 +0,0 @@
-// GoToSocial
-// Copyright (C) GoToSocial Authors admin@gotosocial.org
-// SPDX-License-Identifier: AGPL-3.0-or-later
-//
-// This program is free software: you can redistribute it and/or modify
-// it under the terms of the GNU Affero General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU Affero General Public License for more details.
-//
-// You should have received a copy of the GNU Affero General Public License
-// along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-package api
-
-import (
-	"code.superseriousbusiness.org/gotosocial/internal/api/metrics"
-	"code.superseriousbusiness.org/gotosocial/internal/config"
-	"code.superseriousbusiness.org/gotosocial/internal/middleware"
-	"code.superseriousbusiness.org/gotosocial/internal/router"
-	"github.com/gin-gonic/gin"
-)
-
-type Metrics struct {
-	metrics *metrics.Module
-}
-
-func (mt *Metrics) Route(r *router.Router, m ...gin.HandlerFunc) {
-	if !config.GetMetricsEnabled() {
-		// Noop: metrics
-		// not enabled.
-		return
-	}
-
-	// Create new group on top level "metrics" prefix.
-	metricsGroup := r.AttachGroup("metrics")
-	metricsGroup.Use(m...)
-	metricsGroup.Use(
-		middleware.CacheControl(middleware.CacheControlConfig{
-			// Never cache metrics responses.
-			Directives: []string{"no-store"},
-		}),
-	)
-
-	// Attach basic auth if enabled.
-	if config.GetMetricsAuthEnabled() {
-		var (
-			username = config.GetMetricsAuthUsername()
-			password = config.GetMetricsAuthPassword()
-			accounts = gin.Accounts{username: password}
-		)
-		metricsGroup.Use(gin.BasicAuth(accounts))
-	}
-
-	mt.metrics.Route(metricsGroup.Handle)
-}
-
-func NewMetrics() *Metrics {
-	return &Metrics{
-		metrics: metrics.New(),
-	}
-}
diff --git a/internal/api/metrics/metrics.go b/internal/api/metrics/metrics.go
deleted file mode 100644
index 5161a3b71..000000000
--- a/internal/api/metrics/metrics.go
+++ /dev/null
@@ -1,60 +0,0 @@
-// GoToSocial
-// Copyright (C) GoToSocial Authors admin@gotosocial.org
-// SPDX-License-Identifier: AGPL-3.0-or-later
-//
-// This program is free software: you can redistribute it and/or modify
-// it under the terms of the GNU Affero General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU Affero General Public License for more details.
-//
-// You should have received a copy of the GNU Affero General Public License
-// along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-//go:build !nootel
-
-package metrics
-
-import (
-	"net/http"
-
-	"github.com/gin-gonic/gin"
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promhttp"
-)
-
-type Module struct {
-	handler http.Handler
-}
-
-func New() *Module {
-	// Let prometheus use "identity", ie., no compression,
-	// or "gzip", to match our own gzip compression middleware.
-	opts := promhttp.HandlerOpts{
-		OfferedCompressions: []promhttp.Compression{
-			promhttp.Identity,
-			promhttp.Gzip,
-		},
-	}
-
-	// Instrument handler itself.
-	handler := promhttp.InstrumentMetricHandler(
-		prometheus.DefaultRegisterer,
-		promhttp.HandlerFor(prometheus.DefaultGatherer, opts),
-	)
-
-	return &Module{
-		handler: handler,
-	}
-}
-
-func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
-	attachHandler(http.MethodGet, "", func(c *gin.Context) {
-		// Defer all "/metrics" handling to prom.
-		m.handler.ServeHTTP(c.Writer, c.Request)
-	})
-}
diff --git a/internal/api/metrics/no_metrics.go b/internal/api/metrics/no_metrics.go
deleted file mode 100644
index 5aa7b722d..000000000
--- a/internal/api/metrics/no_metrics.go
+++ /dev/null
@@ -1,31 +0,0 @@
-// GoToSocial
-// Copyright (C) GoToSocial Authors admin@gotosocial.org
-// SPDX-License-Identifier: AGPL-3.0-or-later
-//
-// This program is free software: you can redistribute it and/or modify
-// it under the terms of the GNU Affero General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU Affero General Public License for more details.
-//
-// You should have received a copy of the GNU Affero General Public License
-// along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-//go:build nootel
-
-package metrics
-
-import (
-	"github.com/gin-gonic/gin"
-)
-
-type Module struct{}
-
-func New() *Module { return &Module{} }
-
-func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
-}