Backpressure threshold.
Maximal number of records queued by the endpoint before the endpoint
is paused by the backpressure mechanism.
For input endpoints, this setting bounds the number of records that have
been received from the input transport but haven't yet been consumed by
the circuit since the circuit, since the circuit is still busy processing
previous inputs.
For output endpoints, this setting bounds the number of records that have
been produced by the circuit but not yet sent via the output transport endpoint
nor stored in the output buffer (see enable_output_buffer
).
Note that this is not a hard bound: there can be a small delay between
the backpressure mechanism is triggered and the endpoint is paused, during
which more data may be queued.
The default is 1 million.