Add ctx in SpanDecorator option

[mmoustai]

I needed to add specific tag from contextKey set in the context on an other middleware before this one.

I wasn't able to access it without adding the context in the decorator.

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[mmoustai]

I signed it!

[googlebot]

CLAs look good, thanks!

[fho]

@rnburn could this be merged?

[fho]

pass ctx also to the streaming interceptors:

Suggested change

	otgrpcOpts.decorator(context.Background(), clientSpan, method, nil, nil, err)
	otgrpcOpts.decorator(cs.Context(), clientSpan, method, nil, nil, err)

[fho]

Suggested change

	otgrpcOpts.decorator(context.Background(), serverSpan, info.FullMethod, nil, nil, err)
	otgrpcOpts.decorator(ss.Context(), serverSpan, info.FullMethod, nil, nil, err)