Add stats about how often a connection is blocked because of the flow control limits

[ddragana]

We need stats for traffic in both direction.

count number of send/receive DATA_BLOCKED frames with a unique value.

Add these stats to:
https://github.com/mozilla/neqo/blob/master/neqo-transport/src/stats.rs

[larseggert]

See #1868
CC @mxinden

[mxinden]

Frame stats have been introduced in #1011.

https://phabricator.services.mozilla.com/D228295 exposed them via Glean.

One can explore the metric here:

https://glam.telemetry.mozilla.org/fog/probe/networking_http_3_quic_frame_count/explore?aggType=avg

count number of send/receive DATA_BLOCKED frames with a unique value.

I am not sure what "unique value" is referring to here. @larseggert do you know?

[larseggert]

DATA_BLOCKED contains the offset at which we're blocked. I guess @ddragana meant to log that offset if it's unique.