Fix behaviour for all metric types when only calls with labels are made

[MrLotU]

Fixes #80 #77

Checklist

• [x] The provided tests still run.
• [x] I've created new tests where needed.
• [x] I've updated the documentation if necessary.

Fixes an internal inconsistency where recording to a summary or histogram with labels would overflow into their main (no labels) versions too. (#77)

Also removes "empty" reports of metrics (#80)