Add flyteadmin server config option to enable grpc histogram metrics

[katrogan]

Tracking issue

#4214

Describe your changes

Enables grpc metrics histograms: https://github.com/grpc-ecosystem/go-grpc-prometheus#histograms
which are high cardinality and are therefore made disabled by default.

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Screenshots

Note to reviewers

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (2edc3bd) 59.52% compared to head (8c6e42a) 59.37%.
Report is 2 commits behind head on master.

❗ Current head 8c6e42a differs from pull request most recent head e1f8997. Consider uploading reports for the commit e1f8997 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4231      +/-   ##
==========================================
- Coverage   59.52%   59.37%   -0.15%     
==========================================
  Files         639      552      -87     
  Lines       54611    39905   -14706     
==========================================
- Hits        32506    23694    -8812     
+ Misses      19543    13878    -5665     
+ Partials     2562     2333     -229     

Flag	Coverage Δ
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Files	Coverage Δ
flyteadmin/pkg/config/config.go	20.00% <ø> (+1.25%)	⬆️
flyteadmin/pkg/config/serverconfig_flags.go	63.46% <100.00%> (+3.08%)	⬆️

... and 574 files with indirect coverage changes

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[katrogan]

PTAL @pingsutw @eapolinario