instrumenting/clientlibs: add process_threads

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 <luca.bruno@coreos.com>

content/docs/instrumenting/writing_clientlibs.md

@@ -345,6 +345,7 @@ all times in unixtime/seconds.
 | `process_resident_memory_bytes`    | Resident memory size in bytes.                         | bytes            |
 | `process_heap_bytes`               | Process heap size in bytes.                            | bytes            |
 | `process_start_time_seconds`       | Start time of the process since unix epoch in seconds. | seconds          |
+| `process_threads`                  | Number of OS threads in the process.             | threads          |
 
 ### Runtime metrics