Add ClientTrace struct for JS tracing

[wallyqs]

This is an alternative implementation to enable tracing of requests in the JetStream context #849, using the same idiom of a struct with callbacks as the httptrace package (https://golang.google.cn/pkg/net/http/httptrace/#ClientTrace).

	js, err := nc.JetStream(&ClientTrace{
		RequestSent: func(subj string, payload []byte) {
			log.Printf(">>> %s: %s", subj, string(payload))
		},
		ResponseReceived: func(subj string, payload []byte, hdr Header) {
			log.Printf("<<< %s: %s", subj, string(payload))
		},
	})

Signed-off-by: Waldemar Quevedo wally@synadia.com

[wallyqs]

If this is ok then will make the change for the natscli, it is a small change: https://github.com/nats-io/natscli/blob/6accd61459f59cbb26e6e14cc24b3606ffc7b426/cli/util.go#L448-L454

[coveralls]

Coverage decreased (-0.04%) to 85.104% when pulling f658a93 on with-client-trace into 5753a5f on main.

[wallyqs]

We could expand this to make them more granular for example, to only trace consumer lookups or for when adding retries make it possible to log an event for when there is a backoff, etc...

[kozlovic]

I like the approach, but of course this is again breaking changes (we had just one in previous PR for the KV purge options). I guess we mentioned that the JS/KV API may change, but hopefully we start to be more stable soon.

[kozlovic]

LGTM

[wallyqs]

This one is ready to merge, follow up PR with the natscli changes here: https://github.com/nats-io/natscli/tree/with-client-trace