Parameterize metrics collector args

[stankevich]

Allows passing extra args to the metrics exporter via the cluster spec to e.g. enable/configure additional metrics collectors.

Example:

---
apiVersion: mysql.presslabs.org/v1alpha1
kind: MysqlCluster
metadata:
  name: example
spec:
  # [...]
  metricsExporterExtraArgs:
    - --collect.info_schema.userstats
    - --collect.perf_schema.file_events
  # [...]

[stankevich]

I just force-pushed a change that renames metricsExporterFlags into metricsExporterExtraArgs to keep the naming scheme consistent with the updates I'm currently working on for #437 (e.g. xbstreamExtraArgs, will PR soon). Updated the title and the description here as well.

Thanks!