add ignoreEmptyDatabases for ignore empty database redis_db_keys* metric

[ktlcove]

I use redis for dev, single redis instance has 100000 databases, but 99% empty
so i add a IgnoreEmptyDatabases arg for ignore all empty databases metrics (db_keys, db_keys_expiring)

I am not a developer, i just edit code do what i want and put code here. maybe someone need this

[oliver006]

Thanks for the PR!

[oliver006]

Two things

1. Code change looks alright - can you add a test for this please?
2. I think we could use a bit better name for the flag. Something along the lines of "don't set key metrics for empty databases". Thoughts?

[codecov]

Codecov Report

Attention: Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.73%. Comparing base (71dbe37) to head (3435505).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
main.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #988      +/-   ##
==========================================
- Coverage   80.78%   80.73%   -0.05%     
==========================================
  Files          18       18              
  Lines        2873     2876       +3     
==========================================
+ Hits         2321     2322       +1     
- Misses        446      448       +2     
  Partials      106      106              

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