Add HTTP Attributes spec (#49)

Add HTTP Attributes spec

This document summarizes the attributes to be collected on the HTTP spans.

Collected attributes need to be mapped to backend-specific known tags/labels when exporting.

Zipkin tags:  https://zipkin.io/public/thrift/v1/zipkinCore.html#Const_HTTP_HOST
Jaeger tags: https://github.com/opentracing/specification/blob/master/semantic_conventions.md
Stackdriver attributes 
Stackdriver labels: https://godoc.org/cloud.google.com/go/trace#pkg-constants

Author: JBD

trace/HTTPAttributes.md

@@ -0,0 +1,39 @@
+# HTTP Attributes
+
+This document summarizes HTTP attributes recorded from HTTP
+requests and responses.
+
+HTTP integrations should set the following attributes on the client
+and server spans. For a server, request represents the incoming request.
+For a client, request represents the outgoing request.
+
+All attributes are optional.
+
+| Attribute name            | Description                 | Example value                   |
+|---------------------------|-----------------------------|---------------------------------|
+| "http.host"               | Request URL host            | "example.com"                   |
+| "http.method"             | Request URL method          | "GET"                           |
+| "http.path"               | Request URL path            | "/users/25f4c31d"               |
+| "http.route"              | Matched request URL route   | "/users/:userID"                |
+| "http.user_agent"         | Request user-agent          | "HTTPClient/1.2"                |
+| "http.status_code"        | Response status code        | 200                             |
+
+Exporters should always export the collected attributes.
+Exporters should map the collected attributes to backend's
+known attributes/labels.
+
+The following table summarizes how OpenCensus attributes maps to the
+known attributes/labels on supported tracing backends.
+
+| OpenCensus attribute      | Zipkin             | Jaeger             | Stackdriver Trace label                    |
+|---------------------------|--------------------|--------------------|--------------------------------------------|
+| "http.host"               | "http.host"        |                    | "trace.cloud.google.com/http/host"         |
+| "http.method"             | "http.method"      | "http.method"      | "trace.cloud.google.com/http/method"       |
+| "http.path"               | "http.path"        |                    |                                            |
+| "http.route"              | "http.route"       |                    |                                            |
+| "http.user_agent"         |                    |                    | "trace.cloud.google.com/http/user_agent"   |
+| "http.status_code"        | "http.status_code" | "http.status_code" | "trace.cloud.google.com/http/status_code"  |
+
+Request body and response size of incoming and outgoing requests should be
+represented as message events. Each redirect should be represented as a
+message event.