fix(gateway): Don't cache 302 responses from the registry.

This affects only the communication with the registry, not with anything
related to sending requests to the implementer's graph or its downstream
services.

A `HEAD` request has no body to cache and a 304 response could have only
been negotiated (with the server) by means of a conditional request header
(e.g., `if-none-match` or `if-modified-since`).

We do NOT want to write to the cache in either of these cases!  Without
avoiding this cache write, we will invalidate the cache, thus causing
subsequent conditional requests (e.g., `If-None-Match: "MD%") to be lacking
content to conditionally request against and necessitating a full
request/response.

packages/apollo-gateway/CHANGELOG.md

@@ -5,6 +5,7 @@
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
 - The default branch of the repository has been changed to `main`.  As this changed a number of references in the repository's `package.json` and `README.md` files (e.g., for badges, links, etc.), this necessitates a release to publish those changes to npm. [PR #4302](https://github.com/apollographql/apollo-server/pull/4302)
+- The cache implementation for the HTTP-fetcher which is used when communicating with the Apollo Registry when the gateway is configured to use [managed federation](https://www.apollographql.com/docs/graph-manager/managed-federation/overview/) will no longer write to its cache when it receives a 304 response as such a response indicates that the cache used to conditionally make the request must already be present. [PR #TODO]()
 
 ## 0.16.9
 

packages/apollo-gateway/src/cache.ts

@@ -31,7 +31,18 @@ export class HttpRequestCache implements CacheManager {
   }
 
   async put(request: Request, response: Response) {
-    let body = await response.text();
+    // A `HEAD` request has no body to cache and a 304 response could have
+    // only been negotiated by using a cached body that was still valid.
+    // Therefore, we do NOT write to the cache in either of these cases.
+    // Without avoiding this, we will invalidate the cache, thus causing
+    // subsequent conditional requests (e.g., `If-None-Match: "MD%") to be
+    // lacking content to conditionally request against and necessitating
+    // a full request/response.
+    if (request.method === "HEAD" || response.status === 304) {
+      return response;
+    }
+
+    const body = await response.text();
 
     this.cache.set(cacheKey(request), {
       body,