Db metrics

[markphelps]

• Add prometheus metrics around db connections
• Update fqdn for cache metrics

[codecov-commenter]

Codecov Report

Merging #314 into master will decrease coverage by 1.90%.
The diff coverage is 67.91%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage   83.18%   81.28%   -1.91%     
==========================================
  Files          15       16       +1     
  Lines         886      999     +113     
==========================================
+ Hits          737      812      +75     
- Misses        116      155      +39     
+ Partials       33       32       -1     

Impacted Files	Coverage Δ
storage/db/migrator.go	25.53% <0.00%> (ø)
storage/db/metrics.go	59.61% <59.61%> (ø)
config/config.go	90.74% <100.00%> (ø)
storage/cache/evaluation.go	76.47% <100.00%> (ø)
storage/cache/flag.go	95.23% <100.00%> (ø)
storage/cache/rule.go	86.95% <100.00%> (ø)
storage/cache/segment.go	95.23% <100.00%> (ø)
storage/db/db.go	91.30% <100.00%> (+12.92%)	⬆️

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