Logging changes like the ones proposed in https://gerrit.wikimedia.org/r/#/c/174452 will introduce the use of a single log channel (logGroup) with varying severity levels. To accompany this MWLoggerLegacyLogger should be able to be configured to only log events of a given level or higher. Since $wgDebugLogGroups already supports the concept of a log group being associated with an array of configuration settings it should be trivial to introduce a 'level' setting that describes the minimum level to emit.

As an associated follow up, the WMF Monolog configuration should also use this level to configure the native level based filtering of most Monolog handlers.