Get rid of string fields in prometheus output

Change-Id: I7cc64c9d62df5a64f7c7027db12576f4888d0103
diff --git a/metadata/service/agent/remote/output/prometheus.yml b/metadata/service/agent/remote/output/prometheus.yml
index 2c242f4..0feb428 100644
--- a/metadata/service/agent/remote/output/prometheus.yml
+++ b/metadata/service/agent/remote/output/prometheus.yml
@@ -6,4 +6,5 @@
           bind:
             address: 0.0.0.0
             port: 9126
+          string_as_label: false
           engine: prometheus