Activator: Add metrics

[akyyy]

/area autoscale
/area monitoring

/kind dev

Expected Behavior

Collect metrics. Add necessary logging. Integrate to the distributed tracing.
Metrics such as activation failures, latency & count will be helpful metrics for debugging issues.

The ask of this issue is to evaluate the metrics to be exposed to contributors, operators and possibly to developers and implement them.

Actual Behavior

No metrics yet.