Add telemetry information for instrumentation/coverage calculation

[MarcoRossignoli]

We should add some telemetry instrumentation logging to be able to understand from logs where we have perf issue.
Interesting data would be:

• size of asm in MB
• time spent in main phases
• hits count per assembly
• ...

We can emit by design or add a flag like --/<> CoverletTelemetry to avoid to pollute too much logs for big projects.

Ref issue #1246 (comment)

cc: @daveMueller @petli @tonerdo