MUL-6741: retire heartbeat index and runtime sampler metrics (#7622)

* perf(metrics): remove runtime sampler series

Co-authored-by: multica-agent <github@multica.ai>

* perf(db): retire runtime heartbeat index

Based on the root-cause analysis and implementation exploration by @importcpp in multica-ai/multica#7599.

Co-authored-by: multica-agent <github@multica.ai>

* fix(metrics): address runtime sampler review

Co-authored-by: multica-agent <github@multica.ai>

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Co-authored-by: Sol-Boy <sol-boy@multica-ai.local>
Co-authored-by: multica-agent <github@multica.ai>