Adds a killswitch called "filterExistingFlagNames". When enabled it will
filter out reported SDK metrics and remove all reported metrics for
names that does not match an exiting feature flag in Unleash.
This have proven critical in the rare case of an SDK that start sending
random flag-names back to unleash, and thus filling up the database. At
some point the database will start slowing down due to the noisy data.
In order to not resolve the flagNames all the time we have added a small
cache (10s) for feature flag names. This gives a small delay (10s) from
flag is created until we start allow metrics for the flag when
kill-switch is enabled. We should probably listen to the event-stream
and use that invalidate the cache when a flag is created.
