Export thread count from process_collector (tikv#401)

This simply retrieves and exports the value of the  field from the
already populated  that CPU, memory and start time are
sourced.

Signed-off-by: Alin Sinpalean <alin.sinpalean@gmail.com>
Signed-off-by: Jan Berktold <jberktold@roblox.com>

src/process_collector.rs

@@ -17,12 +17,12 @@ use crate::proto;
 /// The `pid_t` data type represents process IDs.
 pub use libc::pid_t;
 
-/// Six metrics per ProcessCollector.
-const METRICS_NUMBER: usize = 6;
+/// Seven metrics per ProcessCollector.
+const METRICS_NUMBER: usize = 7;
 
-/// A collector which exports the current state of
-/// process metrics including cpu, memory and file descriptor usage as well as
-/// the process start time for the given process id.
+/// A collector which exports the current state of process metrics including
+/// CPU, memory and file descriptor usage, thread count, as well as the process
+/// start time for the given process id.
 #[derive(Debug)]
 pub struct ProcessCollector {
     pid: pid_t,
@@ -33,6 +33,7 @@ pub struct ProcessCollector {
     vsize: Gauge,
     rss: Gauge,
     start_time: Gauge,
+    threads: Gauge,
 }
 
 impl ProcessCollector {
@@ -100,6 +101,13 @@ impl ProcessCollector {
         .unwrap();
         descs.extend(start_time.desc().into_iter().cloned());
 
+        let threads = Gauge::with_opts(
+            Opts::new("process_threads", "Number of OS threads in the process.")
+                .namespace(namespace.clone()),
+        )
+        .unwrap();
+        descs.extend(threads.desc().into_iter().cloned());
+
         ProcessCollector {
             pid,
             descs,
@@ -109,6 +117,7 @@ impl ProcessCollector {
             vsize,
             rss,
             start_time,
+            threads,
         }
     }
 
@@ -166,6 +175,9 @@ impl Collector for ProcessCollector {
             cpu_total.collect()
         };
 
+        // threads
+        self.threads.set(p.stat.num_threads as f64);
+
         // collect MetricFamilys.
         let mut mfs = Vec::with_capacity(METRICS_NUMBER);
         mfs.extend(cpu_total_mfs);
@@ -174,6 +186,7 @@ impl Collector for ProcessCollector {
         mfs.extend(self.vsize.collect());
         mfs.extend(self.rss.collect());
         mfs.extend(self.start_time.collect());
+        mfs.extend(self.threads.collect());
         mfs
     }
 }
@@ -208,7 +221,7 @@ mod tests {
     fn test_process_collector() {
         let pc = ProcessCollector::for_self();
         {
-            // Six metrics per process collector.
+            // Seven metrics per process collector.
             let descs = pc.desc();
             assert_eq!(descs.len(), super::METRICS_NUMBER);
             let mfs = pc.collect();