[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.

helm-charts/charts/prometheus-blackbox-exporter/values.yaml

Lines 273 to 285 in 0529c10

	## If true, a ServiceMonitor CRD is created for a prometheus operator
	## https://github.com/coreos/prometheus-operator for blackbox-exporter itself
	##
	selfMonitor:
	enabled: false
	additionalMetricsRelabels: {}
	additionalRelabeling: []
	labels: {}
	path: /metrics
	scheme: http
	tlsConfig: {}
	interval: 30s
	scrapeTimeout: 30s

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

Checklist

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