Rewrote IMDS emulator in Go, added unit tests.

sts-agent/compose.yaml

@@ -19,8 +19,8 @@ services:
     volumes:
       - /mnt/ext-storage:/mnt/ext-storage:ro
     environment:
-      GCE_METADATA_HOST: imds-emulator
-      GCE_METADATA_IP: imds-emulator
+      GCE_METADATA_HOST: imds-emulator:8080
+      GCE_METADATA_IP: imds-emulator:8080
     command:
       - --project-id=${PROJECT_ID}
       - --hostname=${HOSTNAME}