fix: Conform metric field type to Singer spec

According to the [Singer
Spec](https://github.com/singer-io/getting-started/blob/master/docs/SYNC_MODE.md#metric-messages),
the correct field name is `type` and not `metric_type`.

singer_sdk/metrics.py

@@ -8,7 +8,7 @@
 import logging
 import logging.config
 import os
-from dataclasses import asdict, dataclass, field
+from dataclasses import dataclass, field
 from pathlib import Path
 from time import time
 from typing import TYPE_CHECKING, Any, Generic, Mapping, TypeVar
@@ -79,7 +79,15 @@ def to_json(self) -> str:
         Returns:
             A JSON object.
         """
-        return json.dumps(asdict(self), default=str)
+        return json.dumps(
+            {
+                "type": self.metric_type,
+                "metric": self.metric.value,
+                "value": self.value,
+                "tags": self.tags,
+            },
+            default=str,
+        )
 
 
 def log(logger: logging.Logger, point: Point) -> None: