Add Velero metrics recording rule

[shubham-pampattiwar]

This PR adds a PrometheusRule which removes the instance label from the metrics and restricts the cardinality.

[codecov-commenter]

Codecov Report

Merging #579 (f9d6026) into master (572a5ff) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #579   +/-   ##
=======================================
  Coverage   37.60%   37.60%           
=======================================
  Files          14       14           
  Lines        2917     2917           
=======================================
  Hits         1097     1097           
  Misses       1737     1737           
  Partials       83       83           

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[openshift-ci]

@shubham-pampattiwar: all tests passed!

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[weshayutin]

+1

[simonpasquier]

/lgtm