[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>

internal/observability/metrics.go

@@ -21,46 +21,32 @@ package observability
 
 import (
 	"context"
-	"errors"
+	"fmt"
 
 	"code.superseriousbusiness.org/gotosocial/internal/config"
 	"code.superseriousbusiness.org/gotosocial/internal/db"
 
 	"github.com/gin-gonic/gin"
 	"github.com/technologize/otel-go-contrib/otelginmetrics"
+	"go.opentelemetry.io/contrib/exporters/autoexport"
 	"go.opentelemetry.io/otel"
-	"go.opentelemetry.io/otel/exporters/prometheus"
 	"go.opentelemetry.io/otel/metric"
 	sdk "go.opentelemetry.io/otel/sdk/metric"
 	"go.opentelemetry.io/otel/sdk/metric/exemplar"
-	"go.opentelemetry.io/otel/sdk/resource"
-	semconv "go.opentelemetry.io/otel/semconv/v1.24.0"
 )
 
-const (
-	serviceName = "GoToSocial"
-)
-
-func InitializeMetrics(db db.DB) error {
+func InitializeMetrics(ctx context.Context, db db.DB) error {
 	if !config.GetMetricsEnabled() {
 		return nil
 	}
 
-	if config.GetMetricsAuthEnabled() {
-		if config.GetMetricsAuthPassword() == "" || config.GetMetricsAuthUsername() == "" {
-			return errors.New("metrics-auth-username and metrics-auth-password must be set when metrics-auth-enabled is true")
-		}
+	r, err := Resource()
+	if err != nil {
+		// this can happen if semconv versioning is out-of-sync
+		return fmt.Errorf("building tracing resource: %w", err)
 	}
 
-	r, _ := resource.Merge(
-		resource.Default(),
-		resource.NewSchemaless(
-			semconv.ServiceName(serviceName),
-			semconv.ServiceVersion(config.GetSoftwareVersion()),
-		),
-	)
-
-	prometheusExporter, err := prometheus.New()
+	mt, err := autoexport.NewMetricReader(ctx)
 	if err != nil {
 		return err
 	}
@@ -68,7 +54,7 @@ func InitializeMetrics(db db.DB) error {
 	meterProvider := sdk.NewMeterProvider(
 		sdk.WithExemplarFilter(exemplar.AlwaysOffFilter),
 		sdk.WithResource(r),
-		sdk.WithReader(prometheusExporter),
+		sdk.WithReader(mt),
 	)
 
 	otel.SetMeterProvider(meterProvider)