feat: cache detected environment's default resource

src/index.ts

@@ -127,6 +127,7 @@ class Logging {
   auth: GoogleAuth;
   options: LoggingOptions;
   projectId: string;
+  detectedResource?: object;
 
   constructor(options?: LoggingOptions) {
     // Determine what scopes are needed.

src/log.ts

@@ -761,21 +761,24 @@ class Log {
           typeof optionsOrCallback === 'function' ? optionsOrCallback : cb;
       const self = this;
 
-      if (!options.resource) {
+      if (options.resource) {
+        if (options.resource.labels) {
+          options.resource.labels = snakeCaseKeys(options.resource.labels);
+        }
+        writeWithResource(options.resource);
+      } else if (this.logging.detectedResource) {
+        writeWithResource(this.logging.detectedResource);
+      } else {
         getDefaultResource(this.logging.auth)
             .then(
                 (resource) => {
+                  this.logging.detectedResource = resource;
                   writeWithResource(resource);
                 },
                 () => {
                   // Ignore errors (the API will speak up if it has an issue).
                   writeWithResource(null);
                 });
-      } else {
-        if (options.resource.labels) {
-          options.resource.labels = snakeCaseKeys(options.resource.labels);
-        }
-        writeWithResource(options.resource);
       }
       function writeWithResource(resource: {}|null) {
         let decoratedEntries;

test/log.ts

@@ -343,6 +343,37 @@ describe('Log', () => {
       log.write(ENTRY, optionsWithResource, done);
     });
 
+    it('should cache a detected resource', done => {
+      const fakeResource = 'test-level-fake-resource';
+
+      fakeMetadata.getDefaultResource = async () => {
+        return fakeResource;
+      };
+
+      log.logging.request = () => {
+        assert.strictEqual(log.logging.detectedResource, fakeResource);
+        done();
+      };
+
+      log.write(ENTRY, done);
+    });
+
+    it('should re-use detected resource', done => {
+      log.logging.detectedResource = 'environment-default-resource';
+
+      fakeMetadata.getDefaultResource = () => {
+        throw new Error('Cached resource was not used.');
+      };
+
+      log.logging.request = config => {
+        assert.strictEqual(
+            config.reqOpts.resource, log.logging.detectedResource);
+        done();
+      };
+
+      log.write(ENTRY, done);
+    });
+
     it('should transform camelcase label keys to snake case', done => {
       const CUSTOM_RESOURCE = {
         labels: {