hubble/relay: flush old flows when the buffer drain timeout is reached

[rolinh]

Draining only 1 flow every time the buffer drain timeout is reached
proves to be insufficient for certain cases. The worst case typically
happens for a request with historic data, some filters and follow-mode.
Using the Hubble CLI, this means requests like this one:

hubble observe --last=100 --follow --pod kube-system/coredns-f9fd979d6-66mfg

The behavior before this patch for a request like this, assuming 250
flows matching the requests are stored in Hubble instances ring buffers
and new flows matching the filter criteria are infrequent, would be to
forward 150 flows, then drain 1 flow to forward every time the drain
timeout is reached. This means that for a buffer size of 100, it would
take at least 100 seconds (with the default buffer drain timeout of 1s)
to drain the buffer (unless enough new flows are received in the
meantime and fill the buffer again).

The new code drains all flows in the queue for which the timestamp is
older than now-bufferDrainTimeout. In other words, this would be the
old behavior:

...
QUEUE FULL draining flow 149
QUEUE FULL draining flow 150
TIMEOUT REACHED
DRAINED 1 flow
TIMEOUT REACHED
DRAINED 1 flow
TIMEOUT REACHED
DRAINED 1 flow
...

And this is the new behavior:

...
QUEUE FULL draining flow 149
QUEUE FULL draining flow 150
TIMEOUT REACHED
DRAINED 100 flow
TIMEOUT REACHED
DRAINED 0 flow
TIMEOUT REACHED
DRAINED 0 flow
...

If new flows are received but not in enough numbers to fill the buffer
within the time window of bufferDrainTimeout, the behavior would look
something like this:

...
QUEUE FULL draining flow 149
QUEUE FULL draining flow 150
TIMEOUT REACHED
DRAINED 100 flow
TIMEOUT REACHED
DRAINED 34 flow
TIMEOUT REACHED
DRAINED 72 flow
...

In effect, this means that a time window of bufferDrainTimeout
(default to 1s) is considered to sort flows before forwarding them to
the client. This dramatically improves the query experience for requests
in follow-mode.

Fixes: #13775

[gandro]

Looks good to me! One small unnecessary import in the unit tests.

[rolinh]

test-me-please