Simplify provider interfaces to set error messages

[jessesuen]

• Rename AnalysisProvider to MetricProvider
• Add message field to measurements
• Simplify provider interface so that it never returns error, and error messages are expected to be set in the measurement.

[codecov]

Codecov Report

Merging #189 into master will increase coverage by 0.05%.
The diff coverage is 95%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
+ Coverage   82.69%   82.74%   +0.05%     
==========================================
  Files          34       35       +1     
  Lines        3189     3181       -8     
==========================================
- Hits         2637     2632       -5     
+ Misses        408      406       -2     
+ Partials      144      143       -1

Impacted Files	Coverage Δ
metricproviders/job/job.go	100% <100%> (ø)
analysis/controller.go	50.64% <100%> (ø)	⬆️
utils/metric/metric.go	100% <100%> (ø)
metricproviders/prometheus/prometheus.go	100% <100%> (ø)
analysis/analysis.go	86.34% <81.81%> (+1.32%)	⬆️

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[dthomson25]

LGTM

[dthomson25]

Do we not have a unit test for executing the resume provider?

https://codecov.io/gh/argoproj/argo-rollouts/pull/189/diff#D11-194

[jessesuen]

Will add a test for missing coverage