go-ipfs should measure peer latency from more subsystems than just the DHT

#### Version information:

go-ipfs version: 0.4.19-
Repo version: 7
System version: amd64/linux
Golang version: go1.11.5

#### Type: enhancement

#### Description:

Currently, go-ipfs records peer latencies (`peerstore.Metrics.RecordLatency(peer.ID, time.Duration)`) from only two sources:
- user triggered ping using `ipfs ping `
- DHT requests (message that will lead to a response, not one off)

This situation lead to `ipfs swarm peers --latency` getting filled by peers with `n/a` latencies, because no latencies has ever been recorded for them. I expect that also leads to some bad decision when the connection manager has to drop some of them.

My point is, go-ipfs should record latencies from more subsystem. Maybe bitswap ?