instrument signald read path for APM metrics

context

we're starting to get decent metrics for when and why sendMessage takes longer
in observing those metrics we've noticed that message sending latency shoots up when there are a bunch of incoming read receipts. we'd like to better understand why. this is a first pass at instrumenting the signald read path to see what takes a long time when processing incoming messages

Edited Aug 17, 2020 by aguestuser

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