feat(runtime): Add pauseMs and runCount metrics to runtime

[CagriYonca]

Current Python GC metrics (gc.collect0/1/2) are object count snapshots — when poll_rate increases, the delta grows proportionally, causing the dashboard to show higher GC pressure even if nothing changed.

Node.js and JVM solve this by exposing pause duration instead: gc.gcPause (Node.js) and gc.<collector>.time (JVM) both measure how long GC actually paused the process. The backend normalises these over a 1-minute window and fires an alert at a fixed ms threshold — independent of poll_rate.

Python's gc module doesn't expose pause duration, so the follow-up PR will add it via gc.callbacks:

• gc.pauseMs— total pause time accumulated between polls, comparable to gc.gcPause
• gc.runCount— number of GC cycles in the same interval, a direct signal for cyclic-reference pressure