Expose memory usage in bytes (#6407)

# Motivation

According to [this slack
thread](https://dfinity.slack.com/archives/CH4CADCJX/p1731504006265059),
by exposing heap and stable memory usage in bytes as metrics named
`heap_memory_bytes` and `stable_memory_bytes`, we automatically get some
alerts on high memory usage.

# Changes

1. Expose metrics.

# Tests

1. Manually at
https://qsgjb-riaaa-aaaaa-aaaga-cai.raw.dskloet-ingress.devenv.dfinity.network/metrics
2. Updated `e2e-test-metrics-present.keys`.

# Todos

- [ ] Add entry to changelog (if necessary).
not necessary

rs/backend/src/stats.rs

@@ -71,6 +71,16 @@ pub fn encode_metrics(w: &mut MetricsEncoder<Vec<u8>>) -> std::io::Result<()> {
         gibibytes(wasm_memory_size_bytes()),
         "Amount of wasm memory used by this canister, in binary gigabytes",
     )?;
+    w.encode_gauge(
+        "stable_memory_bytes",
+        stable_memory_size_bytes() as f64,
+        "Amount of stable memory used by this canister, in bytes",
+    )?;
+    w.encode_gauge(
+        "heap_memory_bytes",
+        wasm_memory_size_bytes() as f64,
+        "Amount of wasm memory used by this canister, in bytes",
+    )?;
     w.encode_gauge(
         "nns_dapp_migration_countdown",
         f64::from(stats.migration_countdown.unwrap_or(0)),

scripts/nns-dapp/e2e-test-metrics-present.keys

@@ -1,6 +1,8 @@
 accounts_count
 hardware_wallet_accounts_count
+heap_memory_bytes
 nns_dapp_migration_countdown
 nns_dapp_stable_memory_size_gib
 nns_dapp_wasm_memory_size_gib
+stable_memory_bytes
 sub_accounts_count