feat(metrics): per-timeline metric for on-demand downloads, remove calls_started histogram (#6834)

refs #6737 

# Problem

Before this PR, on-demand downloads weren't  measured per tenant_id.
This makes root-cause analysis of latency spikes harder, requiring us to
resort to log scraping for

```
{neon_service="pageserver"} |= `downloading on-demand` |= `$tenant_id`
```

which can be expensive when zooming out in Grafana.

Context: https://neondb.slack.com/archives/C033RQ5SPDH/p1707809037868189

# Solution / Changes

- Remove the calls_started histogram
- I did the dilegence, there are only 2 dashboards using this histogram,
    and in fact only one uses it as a histogram, the other just as a
    a counter.
- [Link
1](8115b54d9f/neonprod/dashboards/hkXNF7oVz/dashboard-Z31XmM24k.yaml (L1454)):
`Pageserver Thrashing` dashboard, linked from playbook, will fix.
- [Link
2](8115b54d9f/neonprod/dashboards/CEllzAO4z/dashboard-sJqfNFL4k.yaml (L599)):
one of my personal dashboards, unused for a long time, already broken in
other ways, no need to fix.
- replace `pageserver_remote_timeline_client_calls_unfinished` gauge
with a counter pair
- Required `Clone`-able `IntCounterPair`, made the necessary changes in
the `libs/metrics` crate
-  fix tests to deal with the fallout

A subsequent PR will remove a timeline-scoped metric to compensate.

Note that we don't need additional global counters for the per-timeline
counters affected by this PR; we can use the `remote_storage` histogram
for those, which, conveniently, also include the secondary-mode
downloads, which aren't covered by the remote timeline client metrics
(should they?).

libs/metrics/src/lib.rs

@@ -201,6 +201,11 @@ impl<P: Atomic> GenericCounterPairVec<P> {
     pub fn with_label_values(&self, vals: &[&str]) -> GenericCounterPair<P> {
         self.get_metric_with_label_values(vals).unwrap()
     }
+
+    pub fn remove_label_values(&self, res: &mut [Result<()>; 2], vals: &[&str]) {
+        res[0] = self.inc.remove_label_values(vals);
+        res[1] = self.dec.remove_label_values(vals);
+    }
 }
 
 impl<P: Atomic> GenericCounterPair<P> {
@@ -247,6 +252,15 @@ impl<P: Atomic> GenericCounterPair<P> {
     }
 }
 
+impl<P: Atomic> Clone for GenericCounterPair<P> {
+    fn clone(&self) -> Self {
+        Self {
+            inc: self.inc.clone(),
+            dec: self.dec.clone(),
+        }
+    }
+}
+
 /// Guard returned by [`GenericCounterPair::guard`]
 pub struct GenericCounterPairGuard<P: Atomic>(GenericCounter<P>);