Override grpc_server_ping_without_stream_allowed default to true

[owen-d]

This is a regular pain point that users experience (i.e. #4375). I believe we should change this to true even though it changes the default behavior in the weaveworks server config. Notably, we override this in all our clusters (#4182) and changing the default should ensure better out of the box usage.

[owen-d]

This is similar (identical?) to what was done in Cortex: cortexproject/cortex#4168

[pstibrany]

I believe this was done in #4435.

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