Allow extending instrumentation-undici with custom labels for metrics

[lionskape]

Is your feature request related to a problem? Please describe

Today there is no way to add custom attributes for instrumentation-undici histograms.
But it may be necessary to add some application-specific values

Describe the solution you'd like to see

I want to add a custom "record metric" hook, that will allow to add some extra labels based on request-response pair

Describe alternatives you've considered

Looks like the only one alternative here is to implement own metrics, by forking instrumentation-undici plugin.

Additional context

Imagine a situation, where u want to add some application-specific values, like "cluster name" of destination service or something mapped from response request headers. Or split metric as anonymous / authorized user requests. Or something else.

[pichlermarc]

Hi @lionskape - this feature is blocked for the same reasons as in @opentelemetry/instrumentation-http. See open-telemetry/opentelemetry-js#5135

[lionskape]

@pichlermarc oh, so sad. Looks like there is a very long-living blocker: open-telemetry/opentelemetry-js#4096