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. Similar metrics are already exposed by Go and JVM
libraries, and this has been recently requested for Rust too.
As this metrics is runtime independent, it makes sense to have it
generally uniform across languages and libraries.

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 that currently exist.             | threads          |
 
 ### Runtime metrics