metrics: add WAL checkpoint metrics

Add metrics for the WAL checkpoint pages for each database.

Log output:
```
info: WAL checkpoint {"class":"StandaloneSqliteDatabase","dbName":"data","timestamp":"2024-09-24T14:22:13.957Z","walCheckpoint":{"busy":0,"checkpointed":0,"log":0}}
```

Metric output:
```
sqlite_wal_checkpoint_pages{db="data",type="busy"} 0
sqlite_wal_checkpoint_pages{db="data",type="checkpointed"} 0
sqlite_wal_checkpoint_pages{db="data",type="log"} 0
```

src/database/standalone-sqlite.ts

@@ -2348,7 +2348,9 @@ export class StandaloneSqliteDatabaseWorker {
   }
 
   cleanupWal(dbName: 'core' | 'bundles' | 'data' | 'moderation'): void {
-    this.dbs[dbName].pragma('wal_checkpoint(TRUNCATE)');
+    const walCheckpoint = this.dbs[dbName].pragma('wal_checkpoint(TRUNCATE)');
+
+    return walCheckpoint[0];
   }
 }
 
@@ -2971,7 +2973,25 @@ export class StandaloneSqliteDatabase
   }
 
   async cleanupWal(dbName: WorkerPoolName): Promise<void> {
-    return this.queueWrite(dbName, 'cleanupWal', [dbName]);
+    this.queueWrite(dbName, 'cleanupWal', [dbName]).then((walCheckpoint) => {
+      this.log.info('WAL checkpoint', {
+        dbName,
+        walCheckpoint,
+      });
+
+      metrics.sqliteWalCheckpointPages.set(
+        { db: dbName, type: 'busy' },
+        walCheckpoint.busy,
+      );
+      metrics.sqliteWalCheckpointPages.set(
+        { db: dbName, type: 'log' },
+        walCheckpoint.log,
+      );
+      metrics.sqliteWalCheckpointPages.set(
+        { db: dbName, type: 'checkpointed' },
+        walCheckpoint.checkpointed,
+      );
+    });
   }
 }
 
@@ -3114,8 +3134,8 @@ if (!isMainThread) {
           parentPort?.postMessage(null);
           break;
         case 'cleanupWal':
-          worker.cleanupWal(args[0]);
-          parentPort?.postMessage(undefined);
+          const walCheckpoint = worker.cleanupWal(args[0]);
+          parentPort?.postMessage(walCheckpoint);
           break;
         case 'terminate':
           parentPort?.postMessage(null);

src/metrics.ts

@@ -130,6 +130,12 @@ export const saveMethodsDuplicateCounter = new promClient.Counter({
   labelNames: ['method'],
 });
 
+export const sqliteWalCheckpointPages = new promClient.Gauge({
+  name: 'sqlite_wal_checkpoint_pages',
+  help: 'Number of pages in the WAL',
+  labelNames: ['db', 'type'],
+});
+
 //
 // Block importer metrics
 //