Moving exporter charms to use opslib

This commit also includes external DB configuration option.

Change-Id: Iddb4adfae582ecfc6af2d797716e386420ad1df8
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
diff --git a/installers/charm/mongodb-exporter/config.yaml b/installers/charm/mongodb-exporter/config.yaml
index 8d3703e..206bca5 100644
--- a/installers/charm/mongodb-exporter/config.yaml
+++ b/installers/charm/mongodb-exporter/config.yaml
@@ -41,3 +41,6 @@
     type: string
     description: Name of the cluster issuer for TLS certificates
     default: ""
+  mongodb_uri:
+    type: string
+    description: MongoDB URI (external database)