improve latency by running "SELECT" only if the db is different

[oliver006]

related to #980

[codecov]

Codecov Report

Attention: Patch coverage is 84.09091% with 14 lines in your changes missing coverage. Please review.

Project coverage is 82.06%. Comparing base (f99f69a) to head (15c18f7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
exporter/keys.go	83.13%	10 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #981      +/-   ##
==========================================
- Coverage   82.08%   82.06%   -0.03%     
==========================================
  Files          18       18              
  Lines        2663     2732      +69     
==========================================
+ Hits         2186     2242      +56     
- Misses        390      399       +9     
- Partials       87       91       +4     

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[coveralls]

Pull Request Test Coverage Report for Build 13691305086

Details

• 78 of 88 (88.64%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 85.443%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
exporter/keys.go	73	83	87.95%

Totals
Change from base Build 13078474001:	0.04%
Covered Lines:	2342
Relevant Lines:	2741

---

💛 - Coveralls

[oliver006]

Some rudimentary benchmarking with about 10 keys showed a reduction in latency for a full scrape from about 5s to 3s.
There's still room for improvement but that's a lot more work and I don't have bandwidth for that right now.