Implement Context

[g-easy]

Context is an object that holds information (such as Span and TagMap) specific to an operation (such as an RPC) and propagates it implicitly to child operations, function calls, etc.

Work that needs to be done:

• Implement Context object. Draft implementation of Context. #200
• WithContext: for switching into a Context. Draft implementation of Context. #200
• WithSpan, WithTagMap: for switching information into the Context. WithSpan: puts a Span into the current Context. #208 WithTagMap: puts a TagMap into the current Context. #209
• GetCurrentSpan: to retrieve from Context. Add GetCurrentSpan() and GetSpanFromContext(). #211
• GetCurrentTagMap: to retrieve from Context. Add GetCurrentTagMap() and GetTagMapFromContext(). #218
• API for adding tags. Deferred to Design Tagging API #223.
• Add documentation. Add a user guide for Context. #224
• Add a version of Record() that uses the current TagMap. Add a variant of Record() that uses the currently active TagMap. #225
• Update the gRPC plugin to use the Context API.
• gRPC: propagate context.

[bogdandrutu]

I think everything is done here in order to do a new release.