Support for extracting b3 propagation trace id with 128 bits

[stroem]

When I try to extract a b3 tracing id of len 32 (128 bits) it fails to parse it.

dd-trace-go/ddtrace/tracer/textmap.go

Line 337 in a2e9ddd

ctx.traceID, err = strconv.ParseUint(v, 16, 64)

Reading the specification for the headers it states that the X-B3-TraceId header is encoded as 32 or 16 lower-hex characters.
https://github.com/openzipkin/b3-propagation#identifiers

Would be nice if you can support b3 tracing id with 128 bits

[knusbaum]

Related to #695

[gbbr]

I guess we can close now.

[stroem]

Added issue and PR for supporting for B3 extract for dd-lambda-wrapper: DataDog/datadog-lambda-go#50 (@gbbr, would you like to take a look at it?)

That issue depends on this one. Would be nice if dd-trace/tracer repo can be bumped to make use of this fix :)