Loki: backend: use streaming JSON parser (#47656)

* converter: remove __name__ customization because Loki does not do that

Loki does not handle __name__ in a special way.
for Prometheus, the caller can implement the formatting
by themselves

* converter: change labels-formatting

the labels.String() method does not handle strange values well

* loki: backend: use streaming-json parser

* more idiomatic code

Co-authored-by: Todd Treece <360020+toddtreece@users.noreply.github.com>

* simpler row-length check

* simpler code

Co-authored-by: Todd Treece <360020+toddtreece@users.noreply.github.com>

pkg/util/converter/testdata/prom-vector-frame.json

@@ -14,12 +14,13 @@
             }
           },
           {
-            "name": "up",
+            "name": "Value",
             "type": "number",
             "typeInfo": {
               "frame": "float64"
             },
             "labels": {
+              "__name__": "up",
               "job": "prometheus",
               "instance": "localhost:9090"
             }
@@ -51,14 +52,15 @@
             }
           },
           {
-            "name": "up",
+            "name": "Value",
             "type": "number",
             "typeInfo": {
               "frame": "float64"
             },
             "labels": {
               "job": "node",
-              "instance": "localhost:9100"
+              "instance": "localhost:9100",
+              "__name__": "up"
             }
           }
         ]