Span Status: only set description for ERROR status code.

According to the spec:

> Description MUST only be used with the Error StatusCode value.
> An empty Description is equivalent with a not present one.

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#set-status

This commit updates the `Status` implementation to comply with the spec.

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased](https://github.com/open-telemetry/opentelemetry-python/compare/v0.18b0...HEAD)
+- Status now only sets `description` when `status_code` is set to `StatusCode.ERROR`
+  ([#1673])(https://github.com/open-telemetry/opentelemetry-python/pull/1673)
 
 ### Added
 - Add `max_attr_value_length` support to Jaeger exporter

opentelemetry-api/src/opentelemetry/trace/status.py

@@ -48,10 +48,16 @@ def __init__(
     ):
         self._status_code = status_code
         self._description = None
+
         if description is not None and not isinstance(description, str):
             logger.warning("Invalid status description type, expected str")
-        else:
-            self._description = description
+            return
+
+        if status_code != StatusCode.ERROR:
+            logger.warning("description should only be set when status_code is set to StatusCode.ERROR")
+            return
+
+        self._description = description
 
     @property
     def status_code(self) -> StatusCode:

opentelemetry-api/tests/trace/test_status.py

@@ -29,7 +29,25 @@ def test_constructor(self):
         self.assertEqual(status.description, "unavailable")
 
     def test_invalid_description(self):
-        with self.assertLogs(level=WARNING):
+        with self.assertLogs(level=WARNING) as warning:
             status = Status(description={"test": "val"})  # type: ignore
             self.assertIs(status.status_code, StatusCode.UNSET)
             self.assertEqual(status.description, None)
+            self.assertIn("Invalid status description type, expected str", warning.output[0])
+
+    def test_description_and_non_error_status(self):
+        with self.assertLogs(level=WARNING) as warning:
+            status = Status(status_code=StatusCode.OK, description="status description")
+            self.assertIs(status.status_code, StatusCode.OK)
+            self.assertEqual(status.description, None)
+            self.assertIn("description should only be set when status_code is set to StatusCode.ERROR", warning.output[0])
+
+        with self.assertLogs(level=WARNING) as warning:
+            status = Status(status_code=StatusCode.UNSET, description="status description")
+            self.assertIs(status.status_code, StatusCode.UNSET)
+            self.assertEqual(status.description, None)
+            self.assertIn("description should only be set when status_code is set to StatusCode.ERROR", warning.output[0])
+
+        status = Status(status_code=StatusCode.ERROR, description="status description")
+        self.assertIs(status.status_code, StatusCode.ERROR)
+        self.assertEqual(status.description, "status description")