feat(metering): AI cost multiplier hook for AI drivers (#3898)

Emits ai.cost.multiplier.<driver>.<provider>:<model> before recording AI
usage, so what a model costs to charge is policy an extension owns rather
than a number in core. Nothing listening records the provider cost.

MeteringService.withAiCostMultiplier(driver) returns a view of the service
whose recording paths scale costOverride by the hook's answer; every AI
driver hands that view to its providers, so all of them are covered without
touching provider code.

src/backend/clients/database/MySQLDatabaseClient.ts

@@ -20,7 +20,7 @@
 import { readdirSync, readFileSync } from 'fs';
 import { isAbsolute, resolve as resolvePath } from 'path';
 import { metrics } from '@opentelemetry/api';
-import { createPool, type Pool } from 'mysql2';
+import { createPool, ExecuteValues, type Pool } from 'mysql2';
 import { Span } from '../../util/span.js';
 import { AbstractDatabaseClient, type WriteResult } from './DatabaseClient';
 import { SQLBatcher } from './SQLBatcher.js';
@@ -54,6 +54,8 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
     private replicaPool!: Pool;
     private db!: SQLBatcher;
     private dbReplica!: SQLBatcher;
+    /** Primary pool, SELECT-only: same rows as `db`, without the transaction. */
+    private dbPrimaryRead!: SQLBatcher;
     private configuration = Configuration.SINGLE;
     private shutdownStarted = false;
     private shutdownTimer: ReturnType<typeof setTimeout> | null = null;
@@ -79,6 +81,7 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
         console.log('[mysql] connected to primary');
 
         this.db = this.createPrimaryBatcher(this.primaryPool);
+        this.dbPrimaryRead = this.createPrimaryReadBatcher(this.primaryPool);
 
         if (dbConf.replica) {
             this.replicaPool = this.createPool(dbConf.replica);
@@ -162,7 +165,7 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
         query: string,
         params: unknown[] = [],
     ): Promise<Record<string, unknown>[]> {
-        const result = await this.db.execute(query, params);
+        const result = await this.dbPrimaryRead.execute(query, params);
         if (!result) return [];
         return (result[0] as Record<string, unknown>[]) ?? [];
     }
@@ -201,7 +204,7 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
             await conn.beginTransaction();
             try {
                 for (const { statement, values } of entries) {
-                    await conn.execute(statement, values);
+                    await conn.execute(statement, values as ExecuteValues);
                 }
                 await conn.commit();
             } catch (err) {
@@ -224,7 +227,7 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
 
         // Run both reads in parallel — prefer replica when it returns rows,
         // otherwise fall back to primary to handle replication lag.
-        const primaryPromise = this.db.execute(query, params);
+        const primaryPromise = this.dbPrimaryRead.execute(query, params);
         try {
             const replicaResult = await this.dbReplica.execute(query, params);
             if (
@@ -354,6 +357,22 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
         });
     }
 
+    /**
+     * Reads that must see the primary still only read, so they skip the
+     * batcher's transaction wrapper. Worth a separate batcher because BEGIN and
+     * COMMIT are round trips: against a primary in another region they cost
+     * more than the query.
+     */
+    private createPrimaryReadBatcher(pool: Pool): SQLBatcher {
+        return new SQLBatcher(pool, {
+            maxTimeInQueue: 30,
+            maxBatchSize: 5,
+            poolLabel: 'primary',
+            readOnly: true,
+            acquireTimeoutMs: this.config.database?.acquireTimeoutMs,
+        });
+    }
+
     private createReplicaBatcher(pool: Pool): SQLBatcher {
         return new SQLBatcher(pool, {
             maxTimeInQueue: 10,
@@ -378,6 +397,7 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
             database: dbConf.database ?? 'puter',
         });
         this.db = this.createPrimaryBatcher(this.primaryPool);
+        this.dbPrimaryRead = this.createPrimaryReadBatcher(this.primaryPool);
 
         if (this.configuration === Configuration.SINGLE) {
             this.replicaPool = this.primaryPool;