Addresses feedback from another PR
prometheus#4089 (comment)

Signed-off-by: TJ Hoplock <[email protected]>

Signed-off-by: TJ Hoplock <[email protected]>

This changes the majority of our `Notify()` implementations to set up a
new logger with the group key attached under the key name `group_key`,
and then to use that logger in all subsequent calls to the logger,
including passing it through to further functions that accept loggers
via params.

A few of the notify implementations are more complicated; they either
extract the key later in their `Notify()` implementation or within sub
methods, or even conditionally like with sns. I left those mostly as is
for now, as they seem to be more snow-flake-y.

Signed-off-by: TJ Hoplock <[email protected]>