[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>

vendor/go.opentelemetry.io/contrib/bridges/prometheus/config.go

@@ -0,0 +1,47 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package prometheus // import "go.opentelemetry.io/contrib/bridges/prometheus"
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// config contains options for the producer.
+type config struct {
+	gatherers []prometheus.Gatherer
+}
+
+// newConfig creates a validated config configured with options.
+func newConfig(opts ...Option) config {
+	cfg := config{}
+	for _, opt := range opts {
+		cfg = opt.apply(cfg)
+	}
+
+	if len(cfg.gatherers) == 0 {
+		cfg.gatherers = []prometheus.Gatherer{prometheus.DefaultGatherer}
+	}
+
+	return cfg
+}
+
+// Option sets producer option values.
+type Option interface {
+	apply(config) config
+}
+
+type optionFunc func(config) config
+
+func (fn optionFunc) apply(cfg config) config {
+	return fn(cfg)
+}
+
+// WithGatherer configures which prometheus Gatherer the Bridge will gather
+// from. If no registerer is used the prometheus DefaultGatherer is used.
+func WithGatherer(gatherer prometheus.Gatherer) Option {
+	return optionFunc(func(cfg config) config {
+		cfg.gatherers = append(cfg.gatherers, gatherer)
+		return cfg
+	})
+}