Add telemetry event for index updates

[JohnMcPMS]

Change

Adds a telemetry event to collect data on index update frequency. This data will be used to help us optimize default update check interval and for a potential future feature of delta index updates. It is designed with that feature in mind, including some fields that will only be set differently when/if it is implemented.

The collected data is:

Field	Type	Nullable	Description
SourceId	string	No	Identifier of the source being updated (for per-source stratification)
PreviousIndexPublishedAt	datetime	Yes	Timestamp embedded in the previously-held index. Null for net-new clients (no prior index).
NewIndexPublishedAt	datetime	No	Timestamp embedded in the newly-downloaded index.
IndexStalenessDays	float	Yes	(NewIndexPublishedAt − PreviousIndexPublishedAt).TotalDays. Pre-computed for pipeline convenience. Null when PreviousIndexPublishedAt is null.
IsNewClient	bool	No	True when PreviousIndexPublishedAt is null. Pre-computed; simplifies pipeline filtering for net-new client cohort.
UsedDeltaDownload	bool	No	True if the client downloaded a delta file rather than a full index. False for non-delta-capable clients or when no delta was available.
PreviousBaselinePublishedAt	datetime	Yes	Timestamp of the baseline the client held before this update. Null if client had no baseline (non-delta, or net-new delta client).
NewBaselinePublishedAt	datetime	Yes	Timestamp of the baseline in use after this update. Null if UsedDeltaDownload is false.
BaselineStalenessDays	float	Yes	(NewBaselinePublishedAt − PreviousBaselinePublishedAt).TotalDays. Pre-computed. Null when either baseline timestamp is null.
BaselineUpdated	bool	No	True if the client downloaded a new baseline as part of this update cycle. Pre-computed; avoids requiring consumers to diff baseline timestamps.
DownloadedBytes	long	No	Total bytes transferred for this update (baseline + delta, or full index). Actual bytes on the wire.
IsManualUpdate	bool	No	True if the update was directly user initiated; false if it was done automatically.

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