Lazily initiate guid on DefaultTracer

[yuzawa-san]

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Overview

I was doing some flame graph analysis and noticed that every tracer had a guid created. However, it would appear that the guid is only really needed on demand if the trace is being reported on.

Here are the flame/icicle graphs of memory allocations from a simple Spring webflux app. Note the number of instances with byte[], char[], String, and DefaultTracer are most allocated.

Unrelated (for a different issue), the reactive streams instrumentation on spring boot creates an gigantic number of tracers. I'm thinking of ways to maybe cut down on that. Or maybe DefaultTracer/AbstractTracer could be make lighter (there are a lot of fields).

NOTE: does this need to be thread safe? I could add a lock or CAS, if you want.

Related Github Issue

none

Testing

Normal CI suite should pass. If you run flame chart analysis the additional memory allocations should disappear.

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