feat(api): Add metrics for jsonrpsee subscriptions (#733)

## What ❔

A small follow-up PR for #693 that introduces some metrics allowing to
track the health of the new approach. Also, does minor refactoring for
the API server builder so it's more typesafe.

## Why ❔

Observability and maintainability.

## Checklist

- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [x] Tests for the changes have been added / updated.
- [x] Documentation comments have been added / updated.
- [x] Code has been formatted via `zk fmt` and `zk lint`.
- [x] Spellcheck has been run via `cargo spellcheck
--cfg=./spellcheck/era.cfg --code 1`.

core/lib/zksync_core/src/api_server/web3/metrics.rs

@@ -184,12 +184,27 @@ pub(super) enum SubscriptionType {
 #[derive(Debug, Metrics)]
 #[metrics(prefix = "api_web3_pubsub")]
 pub(super) struct PubSubMetrics {
+    /// Latency to load new events from Postgres before broadcasting them to subscribers.
     #[metrics(buckets = Buckets::LATENCIES)]
     pub db_poll_latency: Family<SubscriptionType, Histogram<Duration>>,
+    /// Latency to send an atomic batch of events to a single subscriber.
     #[metrics(buckets = Buckets::LATENCIES)]
     pub notify_subscribers_latency: Family<SubscriptionType, Histogram<Duration>>,
+    /// Total number of events sent to all subscribers of a certain type.
     pub notify: Family<SubscriptionType, Counter>,
+    /// Number of currently active subscribers split by the subscription type.
     pub active_subscribers: Family<SubscriptionType, Gauge>,
+    /// Lifetime of a subscriber of a certain type.
+    #[metrics(buckets = Buckets::LATENCIES)]
+    pub subscriber_lifetime: Family<SubscriptionType, Histogram<Duration>>,
+    /// Current length of the broadcast channel of a certain type. With healthy subscribers, this value
+    /// should be reasonably low.
+    pub broadcast_channel_len: Family<SubscriptionType, Gauge<usize>>,
+    /// Number of skipped broadcast messages.
+    #[metrics(buckets = Buckets::exponential(1.0..=128.0, 2.0))]
+    pub skipped_broadcast_messages: Family<SubscriptionType, Histogram<u64>>,
+    /// Number of subscribers dropped because of a send timeout.
+    pub subscriber_send_timeouts: Family<SubscriptionType, Counter>,
 }
 
 #[vise::register]