Use FastlyMeta for RequestID and SandboxID

[cee-dub]

This makes accessing the RequestID and SandboxID from fsthttp.Request consistent, and enables a move away from reading the FASTLY_TRACE_ID environment variable for SandboxID which is confusing now that we have RequestID.

Users are encouraged to use RequestID() to uniquely identify individual requests being served.

Fixes #183

The RequestID value comes from this host call:

// witx;
//
//	(@interface func (export "downstream_client_request_id")
//	    (param $req $request_handle)
//	    (param $reqid_out (@witx pointer (@witx char8)))
//	    (param $reqid_max_len (@witx usize))
//	    (param $nwritten_out (@witx pointer (@witx usize)))
//	    (result $err (expected (error $fastly_status)))
//	)
//
//go:wasmimport fastly_http_downstream downstream_client_request_id
func fastlyHTTPDownstreamRequestID(...) FastlyStatus

[dgryski]

LGTM

[dgryski]

Merge conflict to be resolved.

[joeshaw]

Could we add request count to FastlyMeta? It's something I would really like to have in my usage.

[cee-dub]

Have to fix conflicts. Seems like a safe add?

[joeshaw]

Opened #215 as a PR onto this branch.

[cee-dub]

I just rebased this. You should be able to merge on top once this goes green again.