[prometheus-snmp-exporter] add selfMonitor

[tkit]

What this PR does / why we need it

Added a selfMonitor configuration to allow the prometheus-operator to monitor the SNMP Exporter itself. This is inspired by the implementation of prometheus-blackbox-exporter and is largely similar in content.

https://github.com/prometheus-community/helm-charts/blob/0529c1011be5ffd26419ef941920368ebdc2ee6f/charts/prometheus-blackbox-exporter/values.yaml#L273-L285 https://github.com/prometheus-community/helm-charts/blob/0529c1011be5ffd26419ef941920368ebdc2ee6f/charts/prometheus-blackbox-exporter/templates/selfservicemonitor.yaml

Checklist

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