Add canary analysis result as Prometheus metrics

[cdlliuy]

Fix of #1147

With this code changes, in my demo case, I can get the following metrics:

# HELP flagger_canary_duration_seconds Seconds spent performing canary analysis.
# HELP flagger_canary_metric_analysis Last canary metric analysis result per different metric template
# TYPE flagger_canary_metric_analysis gauge
flagger_canary_metric_analysis{metricTemplate="flagger-helloworld-demo-error-message-analysis-existing-error-ratio",name="flagger-helloworld-demo-deployment",namespace="ying-cd-poc"} 0.918223108974359
flagger_canary_metric_analysis{metricTemplate="flagger-helloworld-demo-error-message-analysis-new-error",name="flagger-helloworld-demo-deployment",namespace="ying-cd-poc"} 1
flagger_canary_metric_analysis{metricTemplate="flagger-helloworld-demo-http-successful-rate",name="flagger-helloworld-demo-deployment",namespace="ying-cd-poc"} 100

Let me know if there are any testcase are required.

[codecov-commenter]

Codecov Report

Merging #1148 (e0186cb) into main (307813a) will decrease coverage by 0.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1148      +/-   ##
==========================================
- Coverage   57.29%   56.78%   -0.51%     
==========================================
  Files          78       79       +1     
  Lines        6343     6611     +268     
==========================================
+ Hits         3634     3754     +120     
- Misses       2166     2301     +135     
- Partials      543      556      +13     

Impacted Files	Coverage Δ
pkg/controller/scheduler_metrics.go	37.11% <100.00%> (+1.32%)	⬆️
pkg/canary/deployment_controller.go	62.75% <0.00%> (-1.64%)	⬇️
pkg/canary/status.go	48.36% <0.00%> (-0.65%)	⬇️
pkg/controller/scheduler_hooks.go	8.10% <0.00%> (-0.35%)	⬇️
pkg/router/factory.go	0.00% <0.00%> (ø)
pkg/router/gateway_api.go	43.53% <0.00%> (ø)
pkg/router/appmesh_v1beta2.go	88.37% <0.00%> (+0.25%)	⬆️
pkg/router/appmesh.go	85.80% <0.00%> (+0.38%)	⬆️

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[cdlliuy]

@stefanprodan , updated the PR for the suggested changes. Can you help to review again? Thanks!

[stefanprodan]

Please add these metrics to docs here: https://github.com/fluxcd/flagger/blob/main/docs/gitbook/usage/monitoring.md#metrics

[cdlliuy]

@stefanprodan , updated the PR for the suggested changes. Can you help to review again? Thanks!

[stefanprodan]

LGTM

Thanks @cdlliuy 🏅