Support custom HTTP header names for w3c Trace Context (OpenTracing)

[kevinbader]

Depends on: #281

We already support the w3c Trace Context for OpenTracing-based distributed tracing. This spec requires two HTTP headers to be present:

At a minimum they MUST propagate the traceparent and tracestate headers and guarantee traces are not broken.

However, some implementations use different names for those headers, e.g., x-traceparent or my-company-trace. To support this, we should default to the standard header names but additionally allow to customize them.

Proposal:

$ export TRACEPARENT_HTTP_HEADER_NAME=my-custom-traceparent
$ export TRACESTATE_HTTP_HEADER_NAME=my-custom-tracestate
$ mix phx.server