Configure metrics exporter for loadbalancing when possible

[madaraszg-tulip]

PR Description

Metrics support was turned off in otelcol.exporter.loadbalancing for two reasons:

• metrics support is considered development upstream
• the metrics exporter crashed when configured with the default routing_key = traceID

Solve both issues by supporting metrics only if the specified routing_key supports it and stability level is set to experimental

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[clayton-cornell]

Does this need a doc update in https://grafana.com/docs/alloy/latest/reference/components/otelcol/otelcol.exporter.loadbalancing/ ?

[madaraszg-tulip]

Does this need a doc update in https://grafana.com/docs/alloy/latest/reference/components/otelcol/otelcol.exporter.loadbalancing/ ?

I have now added a doc update to the PR

[ptodev]

Thank you for the PR! If I remember correctly, there were two main problems in the past:

• We had to always send metrics, logs, and traces in OTLP components. #5684 fixed this so that we can only send a subset of those. Unfortunately, it also came with a limitation - a component is bound to the same types of telemetry throughout its lifetime. E.g. it might only be able to work for metrics.
• A component could only have one stability label. This is not a problem anymore - we now have the stability cmd args which can check if certain functionality is experimental.

Fortunately, there are ready solutions to those problems now. Unfortunately, one remaining challenge is that this exporter can change the types of outputs at runtime. It can start by accepting only metrics, then after a config update it may want to only accept traces in a way that's not compatible with any metrics config. I'm not sure what a good way to solve this problem would look like. If you want to try to implement a solution, I'd be happy to review the PR again 😊