[appmesh-prometheus] Too many metrics

**Is your feature request related to a problem?**
Apparently enabling `remote_write` (See https://github.com/aws/eks-charts/pull/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.

Start by reviewing the appmesh-prometheus Helm chart and how its remote_write settings are currently configured. Determine what chart configuration would let users select the metrics sent to New Relic, and consider the issue resolved when that configuration is clearly defined and documented.

Written by the indexing model from the issue text.