Add data schema of benchmark metric for bigquery.

official/benchmark/datastore/schema/benchmark_metric.json

+[
+  {
+    "description": "The ID of the benchmark run, where this metric should tie to.",
+    "mode": "REQUIRED",
+    "name": "run_id",
+    "type": "STRING"
+  },
+  {
+    "description": "The name of the metric, which should be descriptive. E.g. training_loss, accuracy.",
+    "mode": "REQUIRED",
+    "name": "name",
+    "type": "STRING"
+  },
+  {
+    "description": "The unit of the metric. E.g. MB per sec.",
+    "mode": "NULLABLE",
+    "name": "unit",
+    "type": "STRING"
+  },
+  {
+    "description": "The value of the metric.",
+    "mode": "NULLABLE",
+    "name": "value",
+    "type": "FLOAT"
+  },
+  {
+    "description": "The timestamp when the metric is recorded.",
+    "mode": "REQUIRED",
+    "name": "timestamp",
+    "type": "TIMESTAMP"
+  },
+  {
+    "description": "The global step when this metric is recorded.",
+    "mode": "NULLABLE",
+    "name": "global_step",
+    "type": "INTEGER"
+  },
+  {
+    "description": "Free format metadata for the extra information about the metric.",
+    "mode": "REPEATED",
+    "name": "extras",
+    "type": "RECORD",
+    "fields": [
+      {
+        "mode": "NULLABLE",
+        "name": "name",
+        "type": "STRING"
+      },
+      {
+        "mode": "NULLABLE",
+        "name": "value",
+        "type": "STRING"
+      }
+    ]
+  }
+]