Miscellaneous additional semantic convention attribute names

[kbrockhoff]

As I am working on various implementations I have found a number attribute names that are not currently specified or have an existing open issue.

http.user_agent - present in OpenCensus, absolutely required by systems my team is currently working on

iam.user - username value from basic authentication or subject from access/identity token, absolutely required by systems my team is currently working on

iam.role - scope/authorities from access token, not needed by my team but useful for completeness of the specification

message_bus.destination - present in OpenTracing, absolutely required by systems my team is currently working on

Also needed is more standardization of "component" attribute values. This already a issue to standardize on db for database spans. Other needs we have found are:

messaging - asynchronous messaging systems
rpc - instead of grpc or if grpc is kept then also needed is rmi, thrift, etc.
cluster - or maybe streaming for internal cross node communications on streaming platforms

[jmacd]

I agree about the four suggested labels. As for the component question, see #336. I feel that component is not well defined (or needed). OTOH, I'd like to specify a standard attribute for the name and version supplied to named tracers and meters.

[kbrockhoff]

All these items have been addressed by other PRs.