When a node restarts or a peering between two nodes starts over for other
reasons, the internal tracking in the Broker manager resets its state (since
it's per-peering), and thus the message overflow counter. The script layer was
unaware of this, and threw errors when trying to reset the corresponding counter
metric down to zero at sync time.
We now track past buffer overflows via a separate epoch table, using Broker peer
ID comparisons to identify new peerings, and set the counter to the sum of past
and current overflows.
I considered just making this a gauge, but it seems more helpful to be able to
look at a counter to see whether any messages have ever been dropped over the
lifetime of the node process.
As an aside, this now also avoids repeatedly creating the labels vector,
re-using the same one for each metric.
Thanks to @pbcullen for identifying this one!
This hooks into Telemetry::sync() to update Broker-level metrics tracking the
peerings' send buffer state. We do this in the cluster framework so we can label
the resulting metrics with Zeek cluster node names, not Broker's endpoint IDs.
