[prometheus-mysql-exporter] Set GOMAXPROCS and GOMEMLIMIT environment variables based on container resources

[jnoordsij]

What this PR does / why we need it

Set GOMAXPROCS and GOMEMLIMIT environment variables based on container resources. This should reduce potential CPU throttling and OOMKills on containers.

The resourceFieldRef is a very specific Kubernetes directive that is created specifically for passing resource-related values, which rounds up the CPU value to the nearest whole number (e.g. 250m to 1) and passes the memory as a numeric value; so 64Mi would result in the environment variable being set to 67108864. This by design makes it completely compatible with Go's API.

An example is documented within Kubernetes documentation itself: https://kubernetes.io/docs/tasks/inject-data-application/environment-variable-expose-pod-information/#use-container-fields-as-values-for-environment-variables.

Inspired by traefik/traefik-helm-chart#1029.

Which issue this PR fixes

N/A

Special notes for your reviewer

This creates an empty env key for those not setting resource values (and not using existing config secret). This is only a little ugly, but should not be harmful. Alternatively, the could be some combined conditional wrapper around the whole env block to only make it appear if a value is set, but that will look quite complicated and quite prone to (future) errors.

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

[monotek]

Pleas fix linting error

[jkroepke]

Looking at prometheus-operator/prometheus-operator#501 - GOMAXPROCS should not set by that way.

[monotek]

I would also say adding some more generice itteration over an "extraEnv" value would be the way to go.

[desaintmartin]

Is there a case where we do not want this to be applied?