feat(iot): Add shelly exporter

This patch adds a shelly exporter based on json-exporter, collects
various metrics for the shelly plugs and can be used by adding the plugs
as targets.

References:
https://shelly-api-docs.shelly.cloud/
https://github.com/prometheus-community/json_exporter
https://artifacthub.io/packages/helm/prometheus-community/prometheus-json-exporter