loki: generate dataplane-compliant logs dataframes (behind feature flag) (#69909)

* add feature flag

* prepare tests

* enable dataplane-format in loki-backend

pkg/tsdb/loki/testdata_dataplane/matrix_gap.json

@@ -0,0 +1,19 @@
+{
+  "status": "success",
+  "data": {
+    "resultType": "matrix",
+    "result": [
+      {
+        "metric": {},
+        "values": [
+          [1643014453.815, "50"],
+          [1643014753.815, "57"],
+          [1643015053.815, "55"],
+          [1643016853.815, "54"],
+          [1643017153.815, "55"],
+          [1643017453.815, "56"]
+        ]
+      }
+    ]
+  }
+}