Moved prometheus monitoring into flask blueprint

cellxgene_gateway/prometheus.py

@@ -0,0 +1,9 @@
+from prometheus_flask_exporter import PrometheusMetrics
+
+from cellxgene_gateway import __version__
+
+
+def add_metrics(app):
+    metrics = PrometheusMetrics(app)
+    metrics.info("app_info", "Application info", version=__version__)
+    return metrics