Update Getter used for Extraction

[carlosalberto]

From this comment in #424:

Jaeger context cannot be parsed using only the Getter interface. Here's an example of Jaeger context:

uber-trace-id: <some string, similar to W3C trace context>
uberctx-tenancy: test
uberctx-user: Yuri

The trace context has a fixed key name uber-trace-id, but baggage is encoded using variable header names uberctx-{baggage-key}.

Also, the W3C tracestate header may be split into multiple entries (repeated headers).

In the OpenTracing there was an iterator over all headers in order to handle this case, not just a Getter. Consider switching to an iterator-based approach.

[Oberon00]

For the repeated-headers, aspect, see #433 (also discussed in the last SIG Spec call)

[carlosalberto]

Closing this one on behalf of #433