instrumenting/clientlibs: add process_threads (#1964)
instrumenting/clientlibs: add process_threads (#1964)

This reserves a new well-known `process_threads` gauge for client
libraries, in order to expose OS threads count on all instrumented
processes.
As this metrics is language/runtime independent, it makes sense to
be under the `process_` namespace so that libraries can align on
the name (if/when they start exposing this).

Signed-off-by: Luca BRUNO <54b63133bb17564df145c2c274f1bc31eb9aa743@coreos.com>
prometheus/docs,prometheus/docs,prometheus/docs