feat(mobile-ui): Expose frame metrics in avg function (#70233)

Allow querying for the average count of slow and frozen frames.

src/sentry/search/events/datasets/spans_metrics.py

@@ -155,9 +155,9 @@ class SpansMetricsDatasetConfig(DatasetConfig):
                             "span.self_time",
                             fields.MetricArg(
                                 "column",
-                                allowed_columns=constants.SPAN_METRIC_DURATION_COLUMNS.union(
-                                    constants.SPAN_METRIC_BYTES_COLUMNS
-                                ),
+                                allowed_columns=constants.SPAN_METRIC_DURATION_COLUMNS
+                                | constants.SPAN_METRIC_BYTES_COLUMNS
+                                | constants.SPAN_METRIC_COUNT_COLUMNS,
                             ),
                         ),
                     ],