Pass exemplar timestamp explicitly

This is a bit more expensive when you don't pass it:

```
histogram without exemplars
                        time:   [3.1824 ns 3.2017 ns 3.2235 ns]
                        change: [-3.6414% +0.1893% +3.1022%] (p = 0.92 > 0.05)
                        No change in performance detected.

histogram with exemplars (no exemplar passed)
                        time:   [6.2238 ns 6.2487 ns 6.2770 ns]
                        change: [+7.5129% +8.3245% +9.0265%] (p = 0.00 < 0.05)
                        Performance has regressed.

histogram with exemplars (some exemplar passed)
                        time:   [70.907 ns 71.667 ns 72.493 ns]
                        change: [+1.4206% +2.3509% +3.3463%] (p = 0.00 < 0.05)
                        Performance has regressed.
```

It should amortize as one passes the same timestamp in multiple observations.

Author: bobrik

benches/exemplars.rs

@@ -19,7 +19,7 @@ pub fn exemplars(c: &mut Criterion) {
         let histogram = HistogramWithExemplars::<Exemplar>::new(BUCKETS.into_iter().copied());
 
         b.iter(|| {
-            histogram.observe(1.0, None);
+            histogram.observe(1.0, None, None);
         });
     });
 
@@ -28,7 +28,7 @@ pub fn exemplars(c: &mut Criterion) {
         let exemplar = vec![("TraceID".to_owned(), "deadfeed".to_owned())];
 
         b.iter(|| {
-            histogram.observe(1.0, Some(exemplar.clone()));
+            histogram.observe(1.0, Some(exemplar.clone()), None);
         });
     });
 }

src/metrics/exemplar.rs

@@ -250,7 +250,7 @@ impl<S> HistogramWithExemplars<S> {
 
     /// Observe the given value, optionally providing a label set and thus
     /// setting the [`Exemplar`] value.
-    pub fn observe(&self, v: f64, label_set: Option<S>) {
+    pub fn observe(&self, v: f64, label_set: Option<S>, timestamp: Option<SystemTime>) {
         let mut inner = self.inner.write();
         let bucket = inner.histogram.observe_and_bucket(v);
         if let (Some(bucket), Some(label_set)) = (bucket, label_set) {
@@ -259,7 +259,7 @@ impl<S> HistogramWithExemplars<S> {
                 Exemplar {
                     label_set,
                     value: v,
-                    time: SystemTime::now(),
+                    time: timestamp.unwrap_or_else(SystemTime::now),
                 },
             );
         }