[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>

example/config.yaml

@@ -993,38 +993,10 @@ request-id-header: "X-Request-Id"
 # Default: false
 tracing-enabled: false
 
-# String. Set the transport protocol for the tracing system. Can either be "grpc"
-# for OTLP gRPC, or "http" for OTLP HTTP.
-# Options: ["grpc", "http"]
-# Default: "grpc"
-tracing-transport: "grpc"
-
-# String. Endpoint of the trace ingester. When using the gRPC or HTTP based transports,
-# provide the endpoint as a single address/port combination without a protocol scheme.
-# Examples: ["localhost:4317"]
-# Default: ""
-tracing-endpoint: ""
-
-# Bool. Disable TLS for the gRPC and HTTP transport protocols.
-# Default: false
-tracing-insecure-transport: false
-
 # Bool. Enable OpenTelemetry based metrics support.
 # Default: false
 metrics-enabled: false
 
-# Bool. Enable HTTP Basic Authentication for Prometheus metrics endpoint.
-# Default: false
-metrics-auth-enabled: false
-
-# String. Username for Prometheus metrics endpoint.
-# Default: ""
-metrics-auth-username: ""
-
-# String. Password for Prometheus metrics endpoint.
-# Default: ""
-metrics-auth-password: ""
-
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 ##### HTTP CLIENT SETTINGS #####
 ################################