The upstream Synapse Grafana dashboard uses `{{job}}-{{index}}` in its
legends to tell workers apart, but the worker scrape configs only set
instance, worker_id, job and app labels. Without an index label, the
dashboard legends collapsed and workers could not be distinguished.

Derive the index label from the worker id (its per-type index is the
first number in the id, e.g. federation-sender-1, stream-writer-2-events),
matching the index label already set on the main process target.

Fixes #2158
Fixes #2159

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>