Support ability to bootstrap and omit errors when metric-server is not configured in flagger helm chart

[pragmaticivan]

Describe the feature

Because my Prometheus instance requires basic auth, I can't use secrets to configure the metric server.

But because of that, I see multiple errors as described below:

{"level":"error","ts":"2021-11-19T19:53:42.109Z","caller":"flagger/main.go:202","msg":"Metrics server http://prometheus:9090 unreachable running query failed: request failed: Get \"http://prometheus:9090/api/v1/query?query=vector%281%29\": dial tcp: lookup prometheus on [redacted]: no such host","stacktrace":"main.main\n\t/workspace/cmd/flagger/main.go:202\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:225"}

Proposed solution

Remove requirement for the Prometheus server and only support Providers instead.