Add latest version metric

* Add a special handler for latest version info

* Add initial latest version items/triggers to the Zabbix template

src/pgmon.py

@@ -699,6 +699,18 @@ class SimpleHTTPRequestHandler(BaseHTTPRequestHandler):
         if metric_name == "agent_version":
             self._reply(200, VERSION)
             return
+        elif metric_name == "latest_version_info":
+            try:
+                get_latest_version()
+                self._reply(
+                    200,
+                    json.dumps(
+                        {"latest": latest_version, "supported": 1 if release_supported else 0}
+                    ),
+                )
+            except LatestVersionCheckError as e:
+                log.error("Failed to retrieve latest version information: {}".format(e))
+                self._reply(503, "Failed to retrieve latest version info")
 
         # Note: parse_qs returns the values as a list.  Since we always expect
         #       single values, just grab the first from each.