[appmesh-prometheus] Too many metrics

[mconigliaro]

Is your feature request related to a problem?
Apparently enabling remote_write (See #978) and sending all these metrics to New Relic made our bill increase by like 6x.

Is your feature request related to a specific Helm chart, if yes mention name of the chart?
appmesh-prometheus

Describe the solution you'd like
How can we configure the metrics we want? Do we need to make a new PR that allows writing the entire prometheus.yml?

Describe alternatives you've considered
Forking and maintaining our own version of the helm chart sounds annoying.