drain
Drain everything the edge is missing, across all three signals.
Safe to call on every reconnect — the per-signal digest comparison ensures only new records move over the wire, and the CRDT merge makes a resend harmless.
Return
DrainResult.Success with per-signal counts (partial successes report what got through), or DrainResult.Failure if every attempted signal failed.
Parameters
edge
the OTLP-capable backend to drain into.