ResponseManager Tracing implementation

[hannahhoward]

What

Replicate similar span tracking on the ResponseManager side

Implementation

For now, when we receive a new incoming request, we'll create a span from root graphsync context (later we want to find a way to hook this span up to any referenced data transfers, if they have their own tracing context, but we're not gonna try that yet).

Our top level span should last the length of the request until the request is finished. The answer to "when is this" is a bit more complicated than it may seem. There are several paths to the true end of the request in ResponseManager (I just realized how not great this is btw -- perhaps a different ticket):

1. Actual finish of request with final response code sent over network: https://github.com/ipfs/go-graphsync/blob/main/responsemanager/subscriber.go#L46
2. Request cancelled by requestor while paused: https://github.com/ipfs/go-graphsync/blob/main/responsemanager/server.go#L113
3. Request cancelled by requestor while not paused: https://github.com/ipfs/go-graphsync/blob/main/responsemanager/queryexecutor/queryexecutor.go#L134
4. Network error cancels request while paused: https://github.com/ipfs/go-graphsync/blob/main/responsemanager/server.go#L117
5. Network error cancels the request while unpaused: https://github.com/ipfs/go-graphsync/blob/main/responsemanager/queryexecutor/queryexecutor.go#L134

bonus Important red herring path: finishTask. finishTask is called when the query has finished executing, meaning all selector data has been read from disk and queued for send. BUT all data has not been sent

We should also track as individual spans for startTask, finishTask, and each run of the query executor.

[rvagg]

closed via #291