Explore: jump to explore from panels with mixed datasources

- extends handlers for panel menu and keypress 'x'
- in a mixed-datasource panel finds first datasource that supports
  explore and collects its targets
- passes those targets to the found datasource to be serialized for
  explore state
- removed `supportMetrics` and `supportsExplore`
- use datasource metadata instead (set in plugin.json)
- Use angular timeout to wrap url change for explore jump
- Extract getExploreUrl into core/utils/explore

public/app/core/utils/explore.ts

@@ -0,0 +1,52 @@
+import { renderUrl } from 'app/core/utils/url';
+
+/**
+ * Returns an Explore-URL that contains a panel's queries and the dashboard time range.
+ *
+ * @param panel Origin panel of the jump to Explore
+ * @param panelTargets The origin panel's query targets
+ * @param panelDatasource The origin panel's datasource
+ * @param datasourceSrv Datasource service to query other datasources in case the panel datasource is mixed
+ * @param timeSrv Time service to get the current dashboard range from
+ */
+export async function getExploreUrl(
+  panel: any,
+  panelTargets: any[],
+  panelDatasource: any,
+  datasourceSrv: any,
+  timeSrv: any
+) {
+  let exploreDatasource = panelDatasource;
+  let exploreTargets = panelTargets;
+  let url;
+
+  // Mixed datasources need to choose only one datasource
+  if (panelDatasource.meta.id === 'mixed' && panelTargets) {
+    // Find first explore datasource among targets
+    let mixedExploreDatasource;
+    for (const t of panel.targets) {
+      const datasource = await datasourceSrv.get(t.datasource);
+      if (datasource && datasource.meta.explore) {
+        mixedExploreDatasource = datasource;
+        break;
+      }
+    }
+
+    // Add all its targets
+    if (mixedExploreDatasource) {
+      exploreDatasource = mixedExploreDatasource;
+      exploreTargets = panelTargets.filter(t => t.datasource === mixedExploreDatasource.name);
+    }
+  }
+
+  if (exploreDatasource && exploreDatasource.meta.explore) {
+    const range = timeSrv.timeRangeForUrl();
+    const state = {
+      ...exploreDatasource.getExploreState(exploreTargets),
+      range,
+    };
+    const exploreState = JSON.stringify(state);
+    url = renderUrl('/explore', { state: exploreState });
+  }
+  return url;
+}