chore(scale-runners): log failures with same key name as variable name

Copilot pointed out correctly that it could be confusing to log
`failedInstanceCount` using the key `missingInstanceCount` when there is
a variable with that name.

Author: iainlane

lambdas/functions/control-plane/src/scale-runners/scale-up.ts

@@ -444,7 +444,7 @@ export async function scaleUp(payloads: ActionRequestMessageSQS[]): Promise<stri
       logger.warn('Some runners failed to be created, rejecting some messages so the requests are retried', {
         wanted: newRunners,
         got: instances.length,
-        missingInstanceCount: failedInstanceCount,
+        failedInstanceCount,
       });
 
       invalidMessages.push(...messages.slice(0, failedInstanceCount).map(({ messageId }) => messageId));