Capture all request and response headers

[pavolloffay]

We have a requirement to capture all request and response headers. This use case has also come up in different instrumentation libraries in OpenTracing project.

By default the feature should be disabled. There should be also a configuration to specify which headers should be collected e.g. CAPTURE_REQUEST_HEADERS=content-type,foo,bar or all - CAPTURE_REQUEST_HEADERS_ALL=true. The configuration properties are a subject for a change.

I will submit a spec issue to define data semantics for headers. e.g. request.header.foo=value and response.header.foo=value2

Related issues in this project: #776 and #566 (comment)

[trask]

Closing, please re-open once there is semantic convention for this.