A transport that reports each flush on telemetry instead of posting it.
Configured in place of the agent transport - typically in config/test.exs -
it turns a flush into something a test can assert on: it answers
{:ok, 202} and emits [:dd_data_streams, :transport, :captured] with
%{count: 1} measurements and %{url: url, headers: headers, body: body}
metadata, where the body is the exact gzip-compressed MsgPack payload the
agent would have received; :zlib.gunzip/1 reads it back.
It keeps no state: a test attaches a telemetry handler under its own id and
receives only what is flushed while that handler is attached, so concurrent
tests never capture each other's flushes through the transport itself.
Combined with DdDataStreams.Aggregator.flush(:all), which sends every
bucket including the one still being filled, a checkpoint recorded a moment
before is assertable without waiting for the ten-second bucket window.