[DPE-3180] Share secrets to host application (#326)

## Issue
1. Mongos charm needs to be notified of updated username/password
2. URI construction in mongos should consider connections made via the
Unix Domain Socket

## Future PR
Most of the work for `DPE-3180` is done on the `mongos` charm. But the
`mongos` charm relies on the libraries hosted by `mongodb` charm to make
the necessary changes future work:
1. updates libraries
2. shares connection info to the application hosting the subordinate
`mongos` charm
3. integration tests verifying its functiondd

lib/charms/mongodb/v0/config_server_interface.py

@@ -35,7 +35,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 1
+LIBPATCH = 2
 
 
 class ClusterProvider(Object):
@@ -139,6 +139,7 @@ def __init__(
         self.database_requires = DatabaseRequires(
             self.charm,
             relation_name=self.relation_name,
+            relations_aliases=[self.relation_name],
             database_name=self.charm.database,
             extra_user_roles=self.charm.extra_user_roles,
             additional_secret_fields=[KEYFILE_KEY],
@@ -149,11 +150,24 @@ def __init__(
             charm.on[self.relation_name].relation_created,
             self.database_requires._on_relation_created_event,
         )
+
+        self.framework.observe(
+            self.database_requires.on.database_created, self._on_database_created
+        )
         self.framework.observe(
             charm.on[self.relation_name].relation_changed, self._on_relation_changed
         )
         # TODO Future PRs handle scale down
 
+    def _on_database_created(self, event) -> None:
+        if not self.charm.unit.is_leader():
+            return
+
+        logger.info("Database and user created for mongos application")
+        self.charm.set_secret(Config.Relations.APP_SCOPE, Config.Secrets.USERNAME, event.username)
+        self.charm.set_secret(Config.Relations.APP_SCOPE, Config.Secrets.PASSWORD, event.password)
+        self.charm.share_connection_info()
+
     def _on_relation_changed(self, event) -> None:
         """Starts/restarts monogs with config server information."""
         key_file_contents = self.database_requires.fetch_relation_field(
@@ -186,7 +200,6 @@ def _on_relation_changed(self, event) -> None:
             event.defer()
             return
 
-        self.charm.share_uri()
         self.charm.unit.status = ActiveStatus()
 
     # BEGIN: helper functions

lib/charms/mongodb/v1/mongos.py

@@ -52,8 +52,11 @@ class MongosConfiguration:
     @property
     def uri(self):
         """Return URI concatenated from fields."""
-        hosts = [f"{host}:{self.port}" for host in self.hosts]
-        hosts = ",".join(hosts)
+        # mongos using Unix Domain Socket to communicate do not use port
+        if self.port:
+            self.hosts = [f"{host}:{self.port}" for host in self.hosts]
+
+        hosts = ",".join(self.hosts)
         # Auth DB should be specified while user connects to application DB.
         auth_source = ""
         if self.database != "admin":