pkg/metrics: Return Service so user can make changes

[lilic]

As per @joelanford suggestion -> #1037 (comment), there are times a user might want to make changes to the Service object before its being created, we should consider returning the object first before creating it, and do the creation in the main.go file instead of the operator-sdk metrics package.

Discussion is needed for this, as it breaks the API.

[lilic]

As part of this we also want to:

• change the name of the Service to append the -metrics string to it, so it's self descriptive
• pass port and name to be able to add more /metrics endpoints.

[lilic]

• change the name of the Service to append the -metrics string to it, so it's self descriptive
• pass port and name to be able to add more /metrics endpoints.

These two are done.

As decided after some discussion, we do not want to create the Service in the user code, so closing this one. If users want we can expose one of the other helper functions.