Return dummy metrics if ClickHouse host or URL is not set in Rotor

[jberlyn]

Problem

When running without ClickHouse (e.g. local dev without the clickhouse service), rotor crashes on startup with CLICKHOUSE_HOST is not defined. This happens because createMetrics() unconditionally calls getClickhouseConfig(), which throws when neither CLICKHOUSE_HOST nor CLICKHOUSE_URL is set.

This issue was raised by someone here:
#1243

Fix

eventsLogger already has a fallback for this case — it uses DummyEventsStore when ClickHouse env vars are absent (src/index.ts:77-81). This PR applies the same pattern to metrics by adding an early return in createMetrics() that yields the already-exported DummyMetrics when ClickHouse isn't configured.

AI Usage Disclosure

I used Claude Code to analyze the rotor codebase and identify the root cause and the existing pattern used for eventsLogger. The fix and this description are my own.