feat: add micrometer compatibility workflow

[zeitlinger]

Summary

Adds the Micrometer compatibility workflow and task tooling that were split out
of prometheus#2093.

This branch is intentionally stacked on top of the extracted prom-side fix:
prometheus#2100

What changed

• Add downstream Micrometer compatibility tasks and workflow so this class of
  regression is exercised automatically against Micrometer
• Support fork-based compatibility testing
• Derive the Micrometer test version from the pom instead of duplicating it
• Pin the downstream Micrometer target to an immutable commit SHA so PR CI is
  deterministic

Related Micrometer work

While debugging the suffix regression, we also found a separate Micrometer
adapter bug: registration-time collision detection needed snapshot-derived
MultiCollector metadata.

That fix lives on the downstream Micrometer branch used by the compatibility
job. The coverage added here is regression protection; the prom-side bug fix is
in prometheus#2100.

Related

• Extracted from: fix: restore OM1/protobuf micrometer suffix compatibility prometheus/client_java#2093
• Stacked on: fix: restore legacy suffix compatibility prometheus/client_java#2100
• Issue: bug: restore OM1/protobuf suffix compatibility after #1955 prometheus/client_java#2095

Testing

• mise run build
• mise run lint