x/net/http2: add ability to specify priority to http2.Transport.RoundTrip

[jared2501]

Example diff: https://github.com/golang/net/pull/17/files

Use case: I'm implementing an HTTP/2 reverse that would like to pass through the priority that the client sends to the upstream server.

[andybons]

/cc @bradfitz

[meirf]

Some notes on the example code (just in case someone uses it as a basis for a CL):

RFC 7540 discusses two ways of sending priority information:

1. include in HEADERS frame
2. in a separate PRIORITY frame

A client can assign a priority for a new stream by including prioritization information in the HEADERS frame (Section 6.2) that opens the stream. At any other time, the PRIORITY frame (Section 6.3) can be used to change the priority of a stream.

It looks like the example code uses option 2:

if priority != nil {
    cc.fr.WritePriority(cs.ID, *priority)
}

but I think what we want is option 1: pass RoundTripOpt.Priority to writeHeaders -> HeadersFrameParam.Priority

Separately, imo the API change (using RoundTripOpt) looks reasonable: minimal and non-breaking.

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Lastly, for the record, there is also the case of sending priority when receiving a promise. See StreamPriority in this comment. But that code hasn't been merged and that case might not be so important so please ignore for the purpose of this issue.