fix: replace environment variable GCE_METADATA_ROOT with GCE_METADATA…

…_HOST

* keeps consistent naming across auth packages of all languages.

    The package will now check GCE_METADATA_HOST (the new name) first; if not present, it falls back to GCE_METADATA_ROOT (the old name), then the default value.

closes [googleapis#339](googleapis#339).

google/auth/compute_engine/_metadata.py

@@ -32,9 +32,16 @@
 
 _LOGGER = logging.getLogger(__name__)
 
-_METADATA_ROOT = "http://{}/computeMetadata/v1/".format(
-    os.getenv(environment_vars.GCE_METADATA_ROOT, "metadata.google.internal")
-)
+# Environment variable GCE_METADATA_HOST is originally named
+# GCE_METADATA_ROOT. For compatiblity reasons, here it checks
+# the new variable first; if not set, the system falls back
+# to the old variable.
+_GCE_METADATA_HOST = os.getenv(environment_vars.GCE_METADATA_HOST, None)
+if not _GCE_METADATA_HOST:
+    _GCE_METADATA_HOST = os.getenv(
+        environment_vars.GCE_METADATA_ROOT, "metadata.google.internal"
+    )
+_METADATA_ROOT = "http://{}/computeMetadata/v1/".format(_GCE_METADATA_HOST)
 
 # This is used to ping the metadata server, it avoids the cost of a DNS
 # lookup.

google/auth/environment_vars.py

@@ -40,9 +40,15 @@
 
 # These two variables allow for customization of the addresses used when
 # contacting the GCE metadata service.
+GCE_METADATA_HOST = "GCE_METADATA_HOST"
 GCE_METADATA_ROOT = "GCE_METADATA_ROOT"
 """Environment variable providing an alternate hostname or host:port to be
-used for GCE metadata requests."""
+used for GCE metadata requests.
+
+This environment variable is originally named GCE_METADATA_ROOT. System will
+check the new variable first; should there be no value present,
+the system falls back to the old variable.
+"""
 
 GCE_METADATA_IP = "GCE_METADATA_IP"
 """Environment variable providing an alternate ip:port to be used for ip-only

tests/compute_engine/test__metadata.py

@@ -155,7 +155,27 @@ def test_get_success_text():
     assert result == data
 
 
-def test_get_success_custom_root():
+def test_get_success_custom_root_new_variable():
+    request = make_request("{}", headers={"content-type": "application/json"})
+
+    fake_root = "another.metadata.service"
+    os.environ[environment_vars.GCE_METADATA_HOST] = fake_root
+    reload_module(_metadata)
+
+    try:
+        _metadata.get(request, PATH)
+    finally:
+        del os.environ[environment_vars.GCE_METADATA_HOST]
+        reload_module(_metadata)
+
+    request.assert_called_once_with(
+        method="GET",
+        url="http://{}/computeMetadata/v1/{}".format(fake_root, PATH),
+        headers=_metadata._METADATA_HEADERS,
+    )
+
+
+def test_get_success_custom_root_old_variable():
     request = make_request("{}", headers={"content-type": "application/json"})
 
     fake_root = "another.metadata.service"