Export ArgoCD stats as prometheus style metrics

[jessesuen]

To better understand an argocd's instance usage (e.g. how frequent deploys/syncs are happening, API requests, how many apps, health states, users, etc...), ArgoCD should export prometheus style metrics to be scraped so it can be used in higher level dashboards.

[jessesuen]

Can use this as a reference implementation:
argoproj/argo-workflows#935

[jessesuen]

First implementation can be limited to health statuses

[merenbach]

Blocked by undefined: proto.InternalMessageInfo errors when running make server and/or make codegen whenever Prometheus client libs are imported. Seems to also occur when switching to master branch and updating either google.golang.org/grpc or github.com/grpc-ecosystem/grpc-gateway.