Change deprecation messaging to be consistently prefixed.

Also some word-smithing to align these similar concepts.

Follows-up: #3923
Follows-up: #3924

packages/apollo-engine-reporting/src/agent.ts

@@ -61,10 +61,10 @@ export function getEngineApiKey(
   const apiKeyFromEnv = process.env.APOLLO_KEY;
 
   if(legacyApiKeyFromEnv && apiKeyFromEnv && !skipWarn) {
-    logger.warn(`Both ENGINE_API_KEY (deprecated) and APOLLO_KEY are set; defaulting to APOLLO_KEY.`);
+    logger.warn("Using `APOLLO_KEY` since `ENGINE_API_KEY` (deprecated) is also set in the environment.");
   }
   if(legacyApiKeyFromEnv && !warnedOnDeprecatedApiKey && !skipWarn) {
-    logger.warn(`[deprecated] Setting the key via ENGINE_API_KEY is deprecated and will not be supported in future versions.`);
+    logger.warn("[deprecated] The `ENGINE_API_KEY` environment variable has been renamed to `APOLLO_KEY`.");
     warnedOnDeprecatedApiKey = true;
   }
   return  apiKeyFromEnv || legacyApiKeyFromEnv || ''
@@ -79,15 +79,15 @@ export function getEngineGraphVariant(engine: EngineReportingOptions<any> | bool
       throw new Error('Cannot set both engine.graphVariant and engine.schemaTag. Please use engine.graphVariant.');
     }
     if (engine.schemaTag) {
-      logger.warn('[Deprecation warning] Usage of engine.schemaTag is deprecated. Please use engine.graphVariant instead.');
+      logger.warn('[deprecated] The `schemaTag` property within `engine` configuration has been renamed to `graphVariant`.');
     }
     return engine.graphVariant || engine.schemaTag;
   } else {
     if (process.env.ENGINE_SCHEMA_TAG) {
-      logger.warn('[Deprecation warning] Usage of ENGINE_SCHEMA_TAG is deprecated. Please use APOLLO_GRAPH_VARIANT instead.');
+      logger.warn('[deprecated] The `ENGINE_SCHEMA_TAG` environment variable has been renamed to `APOLLO_GRAPH_VARIANT`.');
     }
     if (process.env.ENGINE_SCHEMA_TAG && process.env.APOLLO_GRAPH_VARIANT) {
-      throw new Error('Cannot set both ENGINE_SCHEMA_TAG and APOLLO_GRAPH_VARIANT. Please use APOLLO_GRAPH_VARIANT.')
+      throw new Error('`APOLLO_GRAPH_VARIANT` and `ENGINE_SCHEMA_TAG` (deprecated) environment variables must not both be set.')
     }
     return process.env.APOLLO_GRAPH_VARIANT || process.env.ENGINE_SCHEMA_TAG;
   }