Fix bug 1547: Add ingress.class annotation to OSM charms

Change-Id: I436a536929ec5f511721a77a55ee4bb55892d057
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/installers/charm/mysqld-exporter/src/charm.py b/installers/charm/mysqld-exporter/src/charm.py
index 2ae7d83..85a1e67 100755
--- a/installers/charm/mysqld-exporter/src/charm.py
+++ b/installers/charm/mysqld-exporter/src/charm.py
@@ -210,7 +210,7 @@
         # Add ingress resources to PodSpec if site url exists
         if config.site_url:
             parsed = urlparse(config.site_url)
-            annotations = {}
+            annotations = {"kubernetes.io/ingress.class": "public"}
             ingress_resource_builder = IngressResourceV3Builder(
                 f"{self.app.name}-ingress", annotations
             )