Make the channel name shorter for ComponentMetricRequest

The `start_time` is optional, and rarely used (for the time being not
even supported), so there is no need to include it in the channel name
when it is not present at all.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/microgrid/_data_sourcing/_component_metric_request.py

@@ -51,9 +51,12 @@ def get_channel_name(self) -> str:
         Returns:
             A string denoting a channel name.
         """
+        start = f",start={self.start_time}" if self.start_time else ""
         return (
-            f"component_metric_request<namespace={self.namespace},"
+            "component_metric_request<"
+            f"namespace={self.namespace},"
             f"component_id={self.component_id},"
-            f"metric_id={self.metric_id.name},"
-            f"start={self.start_time}>"
+            f"metric_id={self.metric_id.name}"
+            f"{start}"
+            ">"
         )