Throttler: improve log message compatibility with Monolog logger

Throttler: improve log message compatibility with Monolog logger

The `type` log event attribute is used in Wikimedia production logging
to categorize log events by source (e.g. 'mediawiki', 'restbase', etc).
Adding a `type` key to the logging context overwrites the default value
of 'mediawiki'. Rename the key to `throttle` in the context and the
message template.

Change-Id: Ic274159774e43a8749f83c850fff7897956cf377