[feature] Initial metrics

[Tsuribori]

Description

Relating to #1218, this PR implements the following metrics:

• Go performance/runtime metrics
• Bun metrics
• Gin metrics

These are instrumented via OpenTelemetry.

Following settings are added:

	MetricsEnabled:  false,
	MetricsExporter: "prometheus",

Turning metrics on exposes them on a public /metrics endpoint for Prometheus pull.

The code has largely been modeled after #1623

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• 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.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[Tsuribori]

I don't really know if its possible to add tests for this

[tsmethurst]

Thanks for this! Finishing off a couple things now, will try to have a look and review soon!

[daenney]

I think that looks good. This gives us a good starting point and we can start to add custom metrics for things later on. Thanks for the work!

[tsmethurst]

Great work! I'll just rebase this on main (if I can) to see if I can fix the merge conflicts...