Instrument whether Summary includes partial batch chunks

[markfields]

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.~

^ That was wrong - the leading chunks are sent not as part of the batch, but individually (i.e. a bunch of single-message "batches"). So this codepath is certainly hit at scale. And worth instrumenting IMO.

This change adds a stat to Summarizer's telemetryContext indicating how many chunks were added - or remains undefined if none.

After 2.4 ships we can look for this in the logs to find out if this ever happens at scale.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +677 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	460.26 KB	460.4 KB	⯅ +143 Bytes
azureClient.js	558.25 KB	558.4 KB	⯅ +157 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	261.02 KB	261.14 KB	⯅ +122 Bytes
fluidFramework.js	401.41 KB	401.42 KB	⯅ +14 Bytes
loader.js	134.24 KB	134.25 KB	⯅ +14 Bytes
map.js	42.43 KB	42.44 KB	⯅ +7 Bytes
matrix.js	146.58 KB	146.58 KB	⯅ +7 Bytes
odspClient.js	525.54 KB	525.69 KB	⯅ +157 Bytes
odspDriver.js	97.85 KB	97.87 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.81 KB	42.82 KB	⯅ +14 Bytes
sharedString.js	163.3 KB	163.31 KB	⯅ +7 Bytes
sharedTree.js	391.87 KB	391.88 KB	⯅ +7 Bytes
Total Size	3.3 MB	3.3 MB	⯅ +677 Bytes

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Baseline commit: 9e74c7cf115cebc51f888bd0e9019cded2f641e6

Generated by :no_entry_sign: dangerJS against 40c6fe37d61839d1641266390e920e829ee2314d

[microsoft-github-policy-service[bot]]

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