Send stat_tag metric when available

lib/api/middlewares/metrics.js

@@ -50,6 +50,7 @@ function getEventData (req, res, tags) {
         map_id: getLayergroupid({ res }),
         cache_buster: getCacheBuster({ res }),
         template_hash: getTemplateHash({ res }),
+        stat_tag: getStatTag({ res }),
         response_code: res.statusCode.toString(),
         response_time: getResponseTime(res),
         source_domain: req.hostname,
@@ -98,6 +99,17 @@ function getTemplateHash ({ res }) {
     return res.locals.templateHash;
 }
 
+function getStatTag ({ res }) {
+    if (res.locals.mapConfig) {
+        return res.locals.mapConfig.obj().stat_tag;
+    }
+
+    // FIXME: don't expect that mapConfig is already set
+    if (res.locals.mapConfigProvider && res.locals.mapConfigProvider.mapConfig) {
+        return res.locals.mapConfigProvider.mapConfig.obj().stat_tag;
+    }
+}
+
 // FIXME: 'X-Tiler-Profiler' might not be accurate enough
 function getResponseTime (res) {
     const profiler = res.get('X-Tiler-Profiler');