exporter/kafka: add support for defining topic as request metadata

[axw]

Component(s)

exporter/kafka

Is your feature request related to a problem? Please describe.

We have a need to route in tenant-specific Kafka topics, similar to the approach described at https://kafka.apache.org/documentation/#multitenancy-topic-naming.

There are arbitrarily many ways one might want to route data, and these apply to other components, so this is best left to another component -- see #39199 for a proposal. That proposal involves injecting the topic name into request metadata, which can then be referenced by the Kafka exporter.

Describe the solution you'd like

As described in #38888 (comment), I would like to introduce a configuration setting for extracting the topic name from request metadata.

Concretely, we should build on #39204 and introduce additional signal-specific topic_from_metadata settings (alternative name welcome).

Describe alternatives you've considered

Originally I thought of making the topic name templated, as described in #38888.

Additional context

No response

[github-actions]

Pinging code owners:

• exporter/kafka: @pavolloffay @MovieStoreGuy @axw

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[crobert-1]

Issue filed by code owner, removing needs triage

[axw]

I've separated the message key support into #40149

[marclop]

I'll work on this issue to implement it as described.