fix(tracer): fix tag setting on span

[seaerchin]

Problem

we were setting the tags for the trace as a bootstrap process. this means that the tracer.init call does not bootstrap properly (as tracer.use is meant to be called at start-up, prior to actual tracing).

this PR changes it so that the tags are set on the span.

Solution

1. instead of calling tracer.use, which is a bootstrap call, change to get the active span and calling setTag on that span.

Tests

• Find traces in isomer's APM like this one, and verify that the tags are present

[timotheeg]

I suppose we can merge this first since it's meant to be equivalent as what was there before, just cleaner.

But it's still weird to add tags to the scope's span, when we are in the context of a specific axios call.

Since a request handled by express can make multiple axios calls, typically we want to:

1. track the request user itself once at the level of the request's scope, and
2. if we need to track information at the level of the individual axios calls, then they should have their own spans, which would carry their own tags. That, or have log entries with enough structured data.

For 1. there's even a tracer.setUser() utility for that