Fix tornado instrumentation's usage of Span Status (#504)

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://github.com/open-telemetry/opentelemetry-python/compare/v1.2.0-0.21b0...HEAD)
 
+- Fixed cases where description was used with non-error status code when creating Status objects.
+  ([#504](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/504))
+
 ### Added
 - `opentelemetry-instrumentation-botocore` now supports
   context propagation for lambda invoke via Payload embedded headers. 

instrumentation/opentelemetry-instrumentation-tornado/src/opentelemetry/instrumentation/tornado/__init__.py

@@ -95,7 +95,7 @@ from opentelemetry.instrumentation.utils import (
 )
 from opentelemetry.propagate import extract
 from opentelemetry.semconv.trace import SpanAttributes
-from opentelemetry.trace.status import Status
+from opentelemetry.trace.status import Status, StatusCode
 from opentelemetry.util._time import _time_ns
 from opentelemetry.util.http import get_excluded_urls, get_traced_request_attrs
 
@@ -299,10 +299,14 @@ def _finish_span(tracer, handler, error=None):
 
     if ctx.span.is_recording():
         ctx.span.set_attribute(SpanAttributes.HTTP_STATUS_CODE, status_code)
+        otel_status_code = http_status_to_status_code(status_code)
+        otel_status_description = None
+        if otel_status_code is StatusCode.ERROR:
+            otel_status_description = reason
         ctx.span.set_status(
             Status(
-                status_code=http_status_to_status_code(status_code),
-                description=reason,
+                status_code=otel_status_code,
+                description=otel_status_description,
             )
         )