module/apmot: initial tracer implementation

[axw]

Implement an OpenTracing tracer on top of the
core API. Root spans create transactions, and
child spans create spans or transactions,
depending on whether or not the parent is
remote.

Neither baggage nor logging are supported,
nor is the binary propagation format. We use
the (draft) W3C Trace-Context, for which there
is not yet a binary format.

There is a bug in the span context which we
cannot fix until spans are top-level objects,
which will happen with the v2 API. We will
revisit this when the v2 API is supported. The
bug is unlikely to affect anyone, since it will
only occur if you attempt to use span context
after the span has been finished/ended.

In line with the Elastic APM Java agent's
implementation, we support several custom tags:

• type (transaction or span type)
• user.id, user.email, user.username (transaction user context)
• result (transaction result)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@5bb918e). Click here to learn what that means.
The diff coverage is 86.27%.

@@            Coverage Diff            @@
##             master     #173   +/-   ##
=========================================
  Coverage          ?   75.22%           
=========================================
  Files             ?       86           
  Lines             ?     5345           
  Branches          ?        0           
=========================================
  Hits              ?     4021           
  Misses            ?     1131           
  Partials          ?      193

Impacted Files	Coverage Δ
module/apmhttp/traceheaders.go	84.21% <ø> (ø)
module/apmot/context.go	100% <100%> (ø)
module/apmhttp/handler.go	72.5% <100%> (ø)
module/apmhttp/client.go	73.91% <50%> (ø)
module/apmot/tracer.go	83.33% <83.33%> (ø)
module/apmot/span.go	87.74% <87.74%> (ø)

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