SetTag should follow the same convention as LogFields

[MatthewDolan]

Problem

I noticed that the log.Field object borrows from the convention set up by the zap logging library (https://github.com/uber-go/zap) to reduce allocations on logging by having typed functions (e.g. log.String(...) or log.Bool(...)). The SetTag(...) function does not follow that convention though.

Proposal

I would propose that SetTag takes a similar opentracing.Tag object that also reduces allocations.

For example:

span.SetTag(tag.String("hello", "world"))

Questions to address (if any)

There might need to be some path for rollout and backward compatibility.

[yurishkuro]

I think it would've been nice if we did this originally, but at this point it will be a breaking change for SetTag API, which is hard to justify.

[MatthewDolan]

I agree that it doesn't quite make sense to make a user nonbackward-compatible change here.

Maybe adding a new API SetTagTyped. That's a terrible name, but that way it doesn't have impact on consumers. It would just require tracers to upgrade to expose the new API. I feel like changes for tracers are less disruptive than changes for consumers.