[DRAFT] Update name and interface for textmapgetter/textmapsetter

[plantfansam]

When working on #44 (see draft), I learned that ngx.req.set_header does not set response headers. There is also a special method for getting response headers (ngx.resp.get_headers()). To avoid confusion, I renamed text_map_getter and text_map_setter to request_header_getter and request_header_setter.

To maintain symmetry between request_header_* and response_header_*, I also updated the propagators to assume that ngx was passed to them (instead of ngx.req). If we do not change this assumption, then the trace context propagator and baggage propagator would accept ngx.req as a carrier, while the traceresponse propagator would accept ngx.

This would be a breaking change. It's OK if you'd rather not change the existing propagators @yangxikun, just let me know.

[yangxikun]

Seems we need a new "trace context propagator" to extract traceresponse from upstream response header and inject traceresponse to downstream response. So maybe we can keep the trace context propagator and baggage propagator accept ngx.req as a carrier.

Sorry for the quickly say "sure" on #44. After checking opentelemetry specification, I found there was not spec about traceresponse. And traceresponse still in w3c trace context draft, maybe we can wait for the opentelemetry specification to include traceresponse.

Does your service need this feature now?

[plantfansam]

Sorry for the quickly say "sure" on #44. After checking opentelemetry specification, I found there was not spec about traceresponse. And traceresponse still in w3c trace context draft, maybe we can wait for the opentelemetry specification to include traceresponse.

No problem! We do need this feature, but we can implement elsewhere if you do not want it in opentelemetry-lua yet!

[plantfansam]

Seems we need a new "trace context propagator" to extract traceresponse from upstream response header and inject traceresponse to downstream response.

Yes, I have a draft of that here 😄 plantfansam#2. I think it makes sense to keep carrier as ngx.req for the TextMapPropagator.

[plantfansam]

Closing this until we actually want it.