Map faas.name and faas.instance to prometheus job and instance for GMP

[dashpole]

Step 1 for #679.

Since service.name has a default value, we need faas.name to take precedence over service.name. But I still want to give users the option to override job, so i've opted to add job as the override to match other labels.

For instance, i've opted to follow the pattern set by name.

In theory, this could be breaking for users if they are setting service.name or service.instance.id on cloud run and expect it to show up. But if we don't take this approach, this doesn't make the migration to the new value seamless for users that followed the example.

Thoughts? @punya @aabmass

[codecov]

Codecov Report

Merging #695 (c031790) into main (ce6e82d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #695   +/-   ##
=======================================
  Coverage   69.14%   69.14%           
=======================================
  Files          36       36           
  Lines        4728     4728           
=======================================
  Hits         3269     3269           
  Misses       1305     1305           
  Partials      154      154           

Files Changed	Coverage Δ
...ector/googlemanagedprometheus/monitoredresource.go	100.00% <ø> (ø)

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[aabmass]

LGTM and existing tests are passing