more batching to improve checkkeys latency, new memory usage metrics

[oliver006]

more aggressive command batching to reduce roundtrips and improve scrape duration when exporter-to-instance latency is high.

in my local tests this cuts down scrape time for ~50 check keys from 15 seconds to about 5 seconds 1-2(!) seconds

It also adds the key_memory_usage_bytes metric which captures the MEMORY USAGE <<key>> of a key.

[codecov]

Codecov Report

Attention: Patch coverage is 68.04979% with 77 lines in your changes missing coverage. Please review.

Project coverage is 80.78%. Comparing base (9ecfaef) to head (e5bd6a6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
exporter/keys.go	67.91%	56 Missing and 21 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #987      +/-   ##
==========================================
- Coverage   82.11%   80.78%   -1.33%     
==========================================
  Files          18       18              
  Lines        2756     2873     +117     
==========================================
+ Hits         2263     2321      +58     
- Misses        401      446      +45     
- Partials       92      106      +14     

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[coveralls]

Pull Request Test Coverage Report for Build 13869298182

Details

• 185 of 241 (76.76%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.0%) to 84.525%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
exporter/keys.go	184	240	76.67%

Files with Coverage Reduction	New Missed Lines	%
exporter/keys.go	4	78.46%

Totals
Change from base Build 13828328686:	-1.0%
Covered Lines:	2436
Relevant Lines:	2882

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💛 - Coveralls