Rename the 'app' label on metrics to 'appname'

[erkiesken]

This corresponds better to the name used in connstring (appName). Also it will not conflict with common kubernetes labels that often get assigned to app.

Also see: https://www.mongodb.com/docs/manual/reference/connection-string-options/#miscellaneous-configuration https://kubernetes.io/docs/reference/labels-annotations-taints/#app-kubernetes-io-name