Create a Tracer wrapper that provides support for the APIs

[objectiser]

Although some Tracer implementations may wish to natively support the observer and filter APIs, it would be useful to provide a Tracer wrapper that handles the management of these APIs in a tracer provider independent way. As discussed here: #3 (comment)

[sjoerdtalsma]

I think it might be useful to explicitly define (and document) the extension API methods in an interface rather than merely defining a wrapper impl.

That way API extension-support by native tracers can avoid wrapping altogether merely by checking for 'instanceof ' the interface.