Instrument whether Summary includes partial batch chunks #22500
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Description
Based on the fact that inbound batches are always processed synchronously, we don't believe the RemoteMessageProcessor will ever have a partial batch in-flight when Summary happens. But maybe there are cases I'm not thinking of.
This change adds a stat to Summarizer's telemetryContext indicating how many chunks were added - or remains undefined if none.
After 2.3 ships we can look for this in the logs to find out if this ever happens at scale.