[BEAM-14556] Honor the formatter installed on the root handler.

[lukecwik]

This allows for an MDC to be integrated and/or other options when formatting the message.

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[lukecwik]

R: @TheNeuralBit @kileys

[lukecwik]

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[lukecwik]

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[TheNeuralBit]

Should we document somewhere that the way for users to interact with this is a JvmInitializer?

[lukecwik]

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[lukecwik]

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[lukecwik]

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