message: Fix Serializing for Metadata
  This PR changes the field names to better represent the `is_span` and
  `is_event` values being serialized in `Metadata`. Out of band, we noticed the
  previous field names did not correspond with the data we were serializing
  for those 2 functions.
diff: @@ -97,8 +97,8 @@ impl<'a> Serialize for SerializeMetadata<'a> {
  state.serialize_field("file", &self.0.file())?;
  state.serialize_field("line", &self.0.line())?;
  state.serialize_field("fields", &SerializeFieldSet(self.0.fields()))?;
  - state.serialize_field("field_names", &self.0.is_span())?;
  - state.serialize_field("callsite", &self.0.is_event())?;
  + state.serialize_field("is_span", &self.0.is_span())?;
  + state.serialize_field("is_event", &self.0.is_event())?;
  state.end()
  }
  }
  