Emit traceparent header in gateway responses and add sharness test

[iand]

The OTel handler extracts incoming trace headers but does not emit them in responses. This has to be done separately and I chose to add it in our gateway handler. The sharness test verifies that no trace header is emitted unless a trace header was sent with the request, that the emitted header keeps the same trace id and that the trace id is collected in traces.

Note that this is to merge into #9169

[iand]

Needed to avoid a failure in the gotest circleci step that checks whether kubo-as-a-library builds with the current version. Error was

github.com/ipfs/kubo/examples/kubo-as-a-library imports
	github.com/ipfs/kubo/core/coreapi imports
	github.com/ipfs/kubo/tracing imports
	go.opentelemetry.io/otel/exporters/jaeger tested by
	go.opentelemetry.io/otel/exporters/jaeger.test imports
	github.com/stretchr/testify/mock imports
	github.com/stretchr/objx loaded from github.com/stretchr/objx@v0.1.1,
	but go 1.16 would select v0.4.0

[iand]

go-ipfs-http-client failure looks like an unrelated flake

[iand]

sharness failure is in test/sharness/t0310-tracing.sh which is the existing tracing test

[BigLep]

@iand : I saw your mention about engaging on this. I think there is a similar item in bifrost-gateway: ipfs/bifrost-gateway#68 . Ultimately I assume there will be some shared code in Boxo.

[Jorropo]

Do you know the difference with:

		gw = otelhttp.NewHandler(gw, "Gateway.Request", otelhttp.WithPropagators(tracing.Propagator()))

There ?

We already use this for cmds and this works great:

kubo/core/corehttp/commands.go

Line 150 in d1713ca

cmdHandler = otelhttp.NewHandler(cmdHandler, "corehttp.cmdsHandler", otelhttp.WithPropagators(tracing.Propagator()))

They seems equivalent to me, except otelhttp is cleaner and more descriptive.

[hacdias]

I tried that but it didn't set the HTTP headers

[hacdias]

@iand @Jorropo this will be directly included in #9169.