Move log messages to stderr (#635)

As the retrieve script can output the metrics on the standard output, log
messages would pollute the output and complicate scripts that would want to
parse it. Use logging instead of passing stderr to the print statements as
it's mostly the same amount of code.

scripts/retrieve_training_metrics.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 import argparse
+import logging
 import sys
 from os.path import abspath
 
@@ -11,6 +12,10 @@ VERSIONED_URI = "train_{}.{}"
 DATED_VERSIONED_URI = "train_{}.{}.{}"
 BASE_URL = "https://index.taskcluster.net/v1/task/project.relman.bugbug.{}/artifacts/public/metrics.json"
 
+LOGGER = logging.getLogger(__name__)
+
+logging.basicConfig(level=logging.INFO)
+
 
 def main():
     description = "Retrieve a model training metrics"
@@ -33,7 +38,7 @@ def main():
         "--output",
         "-o",
         help="Where to output the metrics.json file. Default to printing its content",
-        default="/dev/stdout",
+        default=None,
     )
 
     args = parser.parse_args()
@@ -46,12 +51,11 @@ def main():
         index_uri = DATED_VERSIONED_URI.format(args.model, args.version, args.date)
 
     index_url = BASE_URL.format(index_uri)
+    LOGGER.info(f"Retrieving metrics from {index_url}")
     r = requests.get(index_url)
 
     if r.status_code == 404:
-        print(
-            f"File not found for URL {index_url}, check your arguments", file=sys.stderr
-        )
+        LOGGER.error(f"File not found for URL {index_url}, check your arguments")
         sys.exit(1)
 
     r.raise_for_status()
@@ -60,7 +64,7 @@ def main():
         file_path = abspath(args.output)
         with open(file_path, "w") as output_file:
             output_file.write(r.text)
-        print(f"Metrics saved to {file_path!r}")
+        LOGGER.info(f"Metrics saved to {file_path!r}")
     else:
         print(r.text)