[BE] Improve autoscaler logging about how many instances are being provisioned (#6619)

The original log could result in lines like: `Available (0) runners will
be below minimum 10. Will provision 0 extra runners` which sounds like
it's not going to provision any instances, but actually what it's trying
to say that the autoscaler will not be overprovisioning any bonus buffer
instances beyond whatever the incoming request was for (i.e. if 5
instances were requested, we'll only be provisioning 5, not more than 5)

Edited the logs to:
1. Clarify that line
2. Only emit that line if we're actually provisioning extra buffer
runners
3. Also emit a log line about the actual number of runners we intend to
provision

This was detected as part of the Chaos Storm testing:
pytorch/pytorch#153155

Author: ZainRizvi

terraform-aws-github-runner/modules/runners/lambdas/runners/src/scale-runners/scale-up.ts

@@ -366,10 +366,12 @@ export function _calculateScaleUpAmount(
     // Never proactively scale up above the minimum limit
     extraScaleUp = Math.min(extraScaleUp, minRunnersUnderprovisionCount);
 
-    console.info(
-      `Available (${availableCount}) runners will be below minimum ${minRunners}. ` +
-        `Will provision ${extraScaleUp} extra runners`,
-    );
+    if (extraScaleUp > 0) {
+      console.info(
+        `Available (${availableCount}) runners will be below minimum ${minRunners}. ` +
+          `Will provision ${extraScaleUp} additional runners above the requested amount to serve as a buffer.`,
+      );
+    }
   }
 
   let scaleUpAmount = extraNeededToAcceptRequests + extraScaleUp;
@@ -382,5 +384,9 @@ export function _calculateScaleUpAmount(
     scaleUpAmount = maxScaleUp;
   }
 
+  console.info(
+    `Will provision a total of ${scaleUpAmount} runners to handle the requested amount of ${requestedCount} runners.`,
+  );
+
   return scaleUpAmount;
 }