Register processor queue length as histogram (#6012)

* Register processor queue length as histogram

* Merge branch 'unstable' of https://github.com/sigp/lighthouse into processor-queue-histogram

common/lighthouse_metrics/src/lib.rs

@@ -283,6 +283,12 @@ pub fn stop_timer(timer: Option<HistogramTimer>) {
     }
 }
 
+pub fn observe_vec(vec: &Result<HistogramVec>, name: &[&str], value: f64) {
+    if let Some(h) = get_histogram(vec, name) {
+        h.observe(value)
+    }
+}
+
 pub fn inc_counter(counter: &Result<IntCounter>) {
     if let Ok(counter) = counter {
         counter.inc();