/metrics.prom counts /follow frames received and dropped, and the relay logs summaryLine() once a minute #247

Split from #110, piece 3 of 3. It builds on the relay's LiveMetrics from piece 1.

Done:

• [ ] The /follow handler counts frames received and frames dropped. Once a minute the relay writes summaryLine() to stdout; the timer is off in tests and is cleared on stop.
• [ ] A relay test that floods /follow shows a dropped count above zero on /metrics.prom.
• [ ] bun x tsc --noEmit and bun --cwd Software/so101 test pass. forge.json is untouched.