Pull Queries: Piggyback lags on every pull query response

[AlanConfluent]

Is your feature request related to a problem? Please describe.
We only report lags on a periodic basis, which means that when servers start up, or if network disruptions occur, we don't have lags. Also, we only update lags ever Xms.

Describe the solution you'd like
If we're doing requests to a host anyway, we should piggyback lags on every pull query so that we have a very precise idea of the current value rather than wait until the next update.

Describe alternatives you've considered
Proactively fetching and caching the lags when unknown. This is likely an easier solution.