[proxy] add metrics for caches (#12752)

Exposes metrics for caches. LKB-2594

This exposes a high level namespace, `cache`, that all cache metrics can
be added to - this makes it easier to make library panels for the caches
as I understand it.

To calculate the current cache fill ratio, you could use the following
query:

```
(
    cache_inserted_total{cache="node_info"}
  - sum (cache_evicted_total{cache="node_info"}) without (cause)
)
  / cache_capacity{cache="node_info"}
```

To calculate the cache hit ratio, you could use the following query:

```
  cache_request_total{cache="node_info", outcome="hit"}
/ sum (cache_request_total{cache="node_info"}) without (outcome)
```

proxy/src/binary/proxy.rs

@@ -535,12 +535,7 @@ pub async fn run() -> anyhow::Result<()> {
     // add a task to flush the db_schema cache every 10 minutes
     #[cfg(feature = "rest_broker")]
     if let Some(db_schema_cache) = &config.rest_config.db_schema_cache {
-        maintenance_tasks.spawn(async move {
-            loop {
-                tokio::time::sleep(Duration::from_secs(600)).await;
-                db_schema_cache.0.run_pending_tasks();
-            }
-        });
+        maintenance_tasks.spawn(db_schema_cache.maintain());
     }
 
     if let Some(metrics_config) = &config.metric_collection {