[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>

cmd/gotosocial/action/server/server.go

@@ -184,7 +184,7 @@ var Start action.GTSAction = func(ctx context.Context) error {
 	}
 
 	// Initialize tracing (noop if not enabled).
-	if err := observability.InitializeTracing(); err != nil {
+	if err := observability.InitializeTracing(ctx); err != nil {
 		return fmt.Errorf("error initializing tracing: %w", err)
 	}
 
@@ -377,7 +377,7 @@ var Start action.GTSAction = func(ctx context.Context) error {
 	}
 
 	// Initialize metrics.
-	if err := observability.InitializeMetrics(state.DB); err != nil {
+	if err := observability.InitializeMetrics(ctx, state.DB); err != nil {
 		return fmt.Errorf("error initializing metrics: %w", err)
 	}
 
@@ -484,7 +484,6 @@ var Start action.GTSAction = func(ctx context.Context) error {
 	var (
 		authModule        = api.NewAuth(state, process, idp, routerSession, sessionName, cookiePolicy) // auth/oauth paths
 		clientModule      = api.NewClient(state, process)                                              // api client endpoints
-		metricsModule     = api.NewMetrics()                                                           // Metrics endpoints
 		healthModule      = api.NewHealth(dbService.Ready)                                             // Health check endpoints
 		fileserverModule  = api.NewFileserver(process)                                                 // fileserver endpoints
 		robotsModule      = api.NewRobots()                                                            // robots.txt endpoint
@@ -538,7 +537,6 @@ var Start action.GTSAction = func(ctx context.Context) error {
 	// apply throttling *after* rate limiting
 	authModule.Route(route, clLimit, clThrottle, robotsDisallowAll, gzip)
 	clientModule.Route(route, clLimit, clThrottle, robotsDisallowAll, gzip)
-	metricsModule.Route(route, clLimit, clThrottle, robotsDisallowAIOnly)
 	healthModule.Route(route, clLimit, clThrottle, robotsDisallowAIOnly)
 	fileserverModule.Route(route, fsMainLimit, fsThrottle, robotsDisallowAIOnly)
 	fileserverModule.RouteEmojis(route, instanceAccount.ID, fsEmojiLimit, fsThrottle, robotsDisallowAIOnly)