Fix adding label matchers for groups if label not in query

[metalmatze]

Pyrra has indeed a bug when it comes to grouping and returning the correct label matchers, but only in a certain case.

Let's take the original issue's example:
grpc_client_handling_seconds_count{job="parca/parca-agent"} for the total metric does NOT contain a grpc_method label matcher but groups by grpc_method.
However, when querying with a specific group matcher like grpc_method=WriteRaw we didn't add that label matcher to the modified query that we want.

With this fix we now get what we want:

-grpc_client_handling_seconds_count{job="parca/parca-agent"}
+grpc_client_handling_seconds_count{job="parca/parca-agent",grpc_method="WriteRaw"}

Thanks for reporting @PabloPie!

Closes #731