Report heap size on /metrics (#2468)

Currently, there is no reporting of the heap size and that metric has
to be obtained from the respective replica dashboard. This PR makes the
heap size more easily accessible so that we can put it onto the Grafana
dashboard too.

src/internet_identity/src/http/metrics.rs

@@ -61,6 +61,12 @@ fn encode_metrics(w: &mut MetricsEncoder<Vec<u8>>) -> std::io::Result<()> {
         stable64_size() as f64,
         "Number of stable memory pages used by this canister.",
     )?;
+    #[cfg(target_arch = "wasm32")]
+    w.encode_gauge(
+        "internet_identity_heap_pages",
+        core::arch::wasm32::memory_size::<0>() as f64,
+        "Number of heap memory pages used by this canister.",
+    )?;
     w.encode_gauge(
         "internet_identity_temp_keys_count",
         state::with_temp_keys(|temp_keys| temp_keys.num_temp_keys()) as f64,

src/internet_identity/tests/integration/http.rs

@@ -145,6 +145,7 @@ fn ii_canister_serves_http_metrics() -> Result<(), CallError> {
         "internet_identity_max_user_number",
         "internet_identity_signature_count",
         "internet_identity_stable_memory_pages",
+        "internet_identity_heap_pages",
         "internet_identity_last_upgrade_timestamp",
         "internet_identity_inflight_challenges",
         "internet_identity_users_in_registration_mode",