adding database and collection stats collection

[DeathBorn]

from both mongos and mongod side with an option to enable/disable collection/database metrics

[DeathBorn]

any news?

[codecov]

Codecov Report

Merging #84 into PMM-1764-small-improvements will decrease coverage by 2.43%.
The diff coverage is 0%.

@@                       Coverage Diff                       @@
##           PMM-1764-small-improvements      #84      +/-   ##
===============================================================
- Coverage                        24.77%   22.33%   -2.44%     
===============================================================
  Files                               35       39       +4     
  Lines                             1990     2207     +217     
===============================================================
  Hits                               493      493              
- Misses                            1473     1688     +215     
- Partials                            24       26       +2

Impacted Files	Coverage Δ
mongodb_exporter.go	1.53% <0%> (-0.03%)	⬇️
collector/mongod/database_status.go	0% <0%> (ø)
collector/mongodb_collector.go	53.33% <0%> (-8.21%)	⬇️
collector/mongod/collections_status.go	0% <0%> (ø)
collector/mongos/collections_status.go	0% <0%> (ø)
collector/mongos/database_status.go	0% <0%> (ø)

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[AlekSi]

I failed to resolve merge conflict in the GitHub UI. Ended up just cherry-picking the commit. Will apply some small fixes in the branch.
Thank you for your contribution! And sorry for a long delay.

[DeathBorn]

@AlekSi welcome :) waiting for merge

[AlekSi]

See #85.