gateway: Introduce Logger type and expose on logger config.

This is a mutually exclusive option to `debug` which enables logging of our
default logger when one is not provided by the user.

Apollo-Orig-Commit-AS: apollographql/apollo-server@780d7f4

gateway-js/src/index.ts

@@ -7,6 +7,7 @@ import {
 import {
   GraphQLExecutionResult,
   GraphQLRequestContext,
+  Logger,
   WithRequired,
 } from 'apollo-server-types';
 import { InMemoryLRUCache } from 'apollo-server-caching';
@@ -19,7 +20,7 @@ import {
 } from 'graphql';
 import { GraphQLSchemaValidationError } from 'apollo-graphql';
 import { composeAndValidate, ServiceDefinition } from '@apollo/federation';
-import loglevel, { Logger } from 'loglevel';
+import loglevel from 'loglevel';
 import loglevelDebug from 'loglevel-debug';
 
 import { buildQueryPlan, buildOperationContext } from './buildQueryPlan';
@@ -46,7 +47,10 @@ import { fetch } from 'apollo-server-env';
 
 export type ServiceEndpointDefinition = Pick<ServiceDefinition, 'name' | 'url'>;
 
-type LoggerConfig = { debug?: boolean; }
+// debug and logger are mutually exclusive options.
+type LoggerConfig =
+  | { debug?: boolean; logger?: never; }
+  | { debug?: never; logger?: Logger; }
 
 type GatewayConfigBase = {
   // TODO: expose the query plan in a more flexible JSON format in the future
@@ -166,7 +170,7 @@ export class ApolloGateway implements GraphQLService {
   public schema?: GraphQLSchema;
   protected serviceMap: DataSourceCache = Object.create(null);
   protected config: GatewayConfig;
-  protected logger: Logger;
+  private logger: Logger;
   protected queryPlanStore?: InMemoryLRUCache<QueryPlan>;
   private engineConfig: GraphQLServiceEngineConfig | undefined;
   private pollingTimer?: NodeJS.Timer;
@@ -214,15 +218,22 @@ export class ApolloGateway implements GraphQLService {
       ...config,
     };
 
-    // Setup logging facilities, scoped under the appropriate name.
-    this.logger = loglevel.getLogger(`apollo-gateway:`);
+    // Setup logging facilities
+    if (this.config.logger) {
+      this.logger = this.config.logger;
+    } else {
+      // If the user didn't prvoide their own logger, we'll initialize one.
+      const loglevelLogger = loglevel.getLogger(`apollo-gateway:`);
 
-    // Support DEBUG environment variable, à la https://npm.im/debug/.
-    loglevelDebug(this.logger);
+      // Support DEBUG environment variable, à la https://npm.im/debug/.
+      loglevelDebug(loglevelLogger);
 
-    // And also support the `debug` option, if it's truthy.
-    if (this.config.debug === true) {
-      this.logger.enableAll();
+      // And also support the `debug` option, if it's truthy.
+      if (this.config.debug === true) {
+        loglevelLogger.enableAll();
+      }
+
+      this.logger = loglevelLogger;
     }
 
     if (isLocalConfig(this.config)) {