TTFB for bitswap shouldn't include candidate collection

[rvagg]

Arising out of this: https://github.com/filecoin-project/lassie/pull/321/files#r1243072856

Discussed today in a call and agreed that we should roughly align it to the other retrievers.

This block:

lassie/pkg/retriever/bitswapretriever.go

Lines 143 to 164 in 46c970b

	bytesWrittenCb := func(bytesWritten uint64) {
	// record first byte received
	if totalWritten.Load() == 0 {
	br.events(events.FirstByte(br.clock.Now(), br.request.RetrievalID, phaseStartTime, bitswapCandidate, br.clock.Since(phaseStartTime)))
	}
	totalWritten.Add(bytesWritten)
	blockCount.Add(1)
	// reset the timer
	if bytesWritten > 0 && lastBytesReceivedTimer != nil {
	lastBytesReceivedTimer.Reset(br.cfg.BlockTimeout)
	}
	}
	// setup providers for this retrieval
	hasCandidates, nextCandidates, err := ayncCandidates.Next(ctx)
	if !hasCandidates || err != nil {
	cancel()
	// we never received any candidates, so we give up on bitswap retrieval
	return nil, nil
	}
	br.events(events.Started(br.clock.Now(), br.request.RetrievalID, phaseStartTime, types.RetrievalPhase, bitswapCandidate, multicodec.TransportBitswap))

The duration in FirstByte is taken from phaseStartTime (startTime in #321) which is at the top of RetrieveFromAsyncCandidates which is prior to collecting the first batch of candidates. The other retrievers take this duration from the beginning of asking the candidate for the data. But for Bitswap it's before we even collect candidates.

Likely solution

The bytesWrittenCb should be moved down lower and a new start clock time recorded before it. That new start time can be used by both the Started event and the FirstByte duration calculation.