Ensure metrics is present before plugin initialization.

This eliminates the need to guard for the presence of `metrics` on the
`requestContext` within plugins who are calling `requestDidStart`.

Ref: #3998 (comment)

packages/apollo-server-core/src/ApolloServer.ts

@@ -869,6 +869,7 @@ export class ApolloServerBase {
       request,
       context: options.context || Object.create(null),
       cache: options.cache!,
+      metrics: {},
       response: {
         http: {
           headers: new Headers(),

packages/apollo-server-core/src/requestPipeline.ts

@@ -126,6 +126,16 @@ export async function processGraphQLRequest<TContext>(
   // all of our own machinery will certainly set it now.
   const logger = requestContext.logger || console;
 
+  // If request context's `metrics` already exists, preserve it, but _ensure_ it
+  // exists there and shorthand it for use throughout this function. As of this
+  // comment, the sole known case where `metrics` already exists is when the
+  // `captureTraces` property is present and set to the result of the boolean
+  // `reporting` option on the legacy (V1) server options, here:
+  // https://git.io/Jfmsb. I suspect this disappears when this is the direct
+  // entry into request processing, rather than through, e.g. `runHttpQuery`.
+  const metrics = requestContext.metrics =
+    requestContext.metrics || Object.create(null);
+
   let cacheControlExtension: CacheControlExtension | undefined;
   const extensionStack = initializeExtensionStack();
   (requestContext.context as any)._extensionStack = extensionStack;
@@ -137,11 +147,6 @@ export async function processGraphQLRequest<TContext>(
 
   await initializeDataSources();
 
-  const metrics = requestContext.metrics || Object.create(null);
-  if (!requestContext.metrics) {
-    requestContext.metrics = metrics;
-  }
-
   const request = requestContext.request;
 
   let { query, extensions } = request;

packages/apollo-server-types/src/index.ts

@@ -104,7 +104,7 @@ export interface GraphQLRequestContext<TContext = Record<string, any>> {
    */
   readonly errors?: ReadonlyArray<GraphQLError>;
 
-  readonly metrics?: GraphQLRequestMetrics;
+  readonly metrics: GraphQLRequestMetrics;
 
   debug?: boolean;
 }