Define additional standard HTTP tags

[yurishkuro]

There is a discussion in https://github.com/census-instrumentation/opencensus-specs/pull/60/files about other HTTP tags beyond the 3 currently defined in OT (url, method, status_code).

Some of their tags may not be necessary (and in fact expensive to get in some instrumentations that only have access to the full URL) because we already capture the full URL. Others might be useful, e.g.

• http.route - the internal mapping defined by the server, such as /api/user/{userID}. Although when this info is available, it is usually used as the span name, but normalizing it as span tag might be useful
• http.user_agent

[SEJeff]

http.host is super useful since neither jaeger or zipkin support querying via a wildcard. My use of jaeger + k8s is for tracing starting at the nginx ingress (kubernetes with nginx-opentracing + jaeger dynamic modules) and being able to filter requests by the http Host header would be a huge improvement.