Merge branch 'jh/trace2-timers-and-counters'

Merge branch 'jh/trace2-timers-and-counters'

Two new facilities, "timer" and "counter", are introduced to the
trace2 API.

* jh/trace2-timers-and-counters:
  trace2: add global counter mechanism
  trace2: add stopwatch timers
  trace2: convert ctx.thread_name from strbuf to pointer
  trace2: improve thread-name documentation in the thread-context
  trace2: rename the thread_name argument to trace2_thread_start
  api-trace2.txt: elminate section describing the public trace2 API
  tr2tls: clarify TLS terminology
  trace2: use size_t alloc,nr_open_regions in tr2tls_thread_ctx