Add the ability to add custom metadata to instances

[pbolduc]

This PR adds the ability to add custom metadata to the instances. The instances are the pods where the postgres exporter runs. This allows adding the correct annotations for sysdig to pull metrics.

my-values.yaml

instances:
  metadata:
    annotations:
      prometheus.io/scrape: 'true'
      prometheus.io/port: '9187'

pgmonitor:
  enabled: true

Deploy: helm upgrade --install sysdig-demo crunchy-postgres --values my-values.yaml

Once deployed, the metrics will show up in sysdig automatically.

and the default dashboards will work, for example,