Add naming pattern for Redis HLL metrics to Telemetry guide

Adding more context to prefix used.

• g_ prefix is from group, as an event which is tracked for group

• p_ is from project as an event which is tracked for project

• i_ from instance as an event which is tracked for instance

• o_ for other

• a_ all (of g_ p_ i_)

This is a pattern which was initially introduced by manage analytics group, I think make sense to use it further