Add Go runtime metrics integration

External PoC for automatic Go runtime health metrics, analogous to Node's `nodeRuntimeMetricsIntegration`.

Raised by Reinaldy Rafli via getsentry/sentry-go#1400. Related prior scoping: getsentry/sentry-go#1157.

* PoC collects runtime signals on a timer (default ~30s) from a single goroutine
* Candidate signals discussed: heap/stack memory, goroutine count, GC pause/count (opt-in), possibly mutex wait (opt-in)
* Naming direction under cross-SDK discussion: generic process metrics as `process.X` (e.g. `process.memory.usage`), Go-specific runtime metrics as `go.X` (e.g. `go.goroutine.count`)
* PoC already exercised in production testing with series such as `go.runtime.mem.*`, `go.runtime.cpu.utilization`, and `go.runtime.goroutines.total`; collection overhead not fully measured yet

Action taken on behalf of Johannes Daxboeck.

Start by reviewing the external PoC and the prior scoping discussions in getsentry/sentry-go#1400 and #1157. Compare the proposed runtime signals, naming direction, timer behavior, opt-in choices, and collection overhead, then define what an accepted integration and its validation would require.

Written by the indexing model from the issue text.