feat: Improve Prometheus metrics

[dthomson25]

Closes #173 and #457

[codecov]

Codecov Report

Merging #461 into master will increase coverage by 0.28%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
+ Coverage   85.17%   85.45%   +0.28%     
==========================================
  Files          80       83       +3     
  Lines        7539     7650     +111     
==========================================
+ Hits         6421     6537     +116     
+ Misses        804      800       -4     
+ Partials      314      313       -1     

Impacted Files	Coverage Δ
controller/metrics/rollouts.go	70.45% <70.45%> (ø)
analysis/controller.go	51.89% <100.00%> (+0.61%)	⬆️
controller/metrics/analysis.go	100.00% <100.00%> (ø)
controller/metrics/client.go	100.00% <100.00%> (ø)
controller/metrics/experiments.go	100.00% <100.00%> (ø)
controller/metrics/metrics.go	100.00% <100.00%> (+20.22%)	⬆️
experiments/controller.go	70.18% <100.00%> (+0.18%)	⬆️
rollout/controller.go	72.56% <100.00%> (ø)
utils/controller/controller.go	80.15% <100.00%> (ø)
... and 2 more

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[jessesuen]

I would remove the creation time metrics as they don't really serve a purpose.