[feature] Initial Prometheus metrics implementation (#2334)

* feat: Initial OTEL metrics

* docs: add metrics documentation

* fix: metrics endpoint conditional check

* feat: metrics endpoint basic auth

* fix: make metrics-auth-enabled default false

* fix: go fmt helpers.gen.go

* fix: add metric-related env vars to envparsing.sh

* fix: metrics docs

* fix: metrics related stuff in envparsing.sh

* fix: metrics docs

* chore: metrics docs wording

* fix: metrics stuff in envparsing?

* bump otel versions

---------

Co-authored-by: Tsuribori <user@acertaindebian>
Co-authored-by: Tsuribori <none@example.org>
Co-authored-by: tsmethurst <tobi.smethurst@protonmail.com>

docs/advanced/index.md

@@ -14,3 +14,4 @@ We consider these topics advanced because applying them incorrectly does have th
 * [Process sandboxing](security/sandboxing.md)
 * [Firewall configuration](security/firewall.md)
 * [Tracing](tracing.md)
+* [Metrics](metrics.md)

docs/advanced/metrics.md

@@ -0,0 +1,37 @@
+# Metrics
+
+GoToSocial comes with [OpenTelemetry][otel] based metrics built-in with pull-style Prometheus exporter. Currently the following metrics are collected:
+
+* Go performance and runtime metrics
+* Gin (HTTP) metrics
+* Bun (database) metrics
+
+How to configure metrics is explained in the [Observability configuration reference][obs].
+
+For a quickstart, add the following to your GoToSocial configuration and restart your instance:
+
+```yaml
+metrics-enabled: true
+metrics-auth-enabled: true
+metrics-auth-username: some_username
+metrics-auth-password: some_password
+```
+
+This will expose the metrics under the endpoint `/metrics`, protected with HTTP Basic Authentication.
+
+A following is an example how to configure a job for collecting the metrics in Prometheus `scrape_configs`:
+
+```yaml
+  - job_name: gotosocial
+    metrics_path: /metrics
+    scheme: https
+    basic_auth:
+        username: some_username
+        password: some_password
+    static_configs:
+    - targets:
+      - example.org
+```
+
+[otel]: https://opentelemetry.io/
+[obs]: ../configuration/observability.md