Investigate Spanner's reported session count

[data-sync-user]

The Spanner Session metric (the number of sessions reported from Spanner itself) radically differs from syncstorage’s own internal count of sessions. We’ve previously had an issue where the session count was too high to the point of it degrading Spanner’s performance, so it’s important that we’re confident in these numbers.

We should contact GCP support for advise and to look into why these metrics don’t line up.

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