message: api: return error code InflightMetrics instead of BlobCacheMetrics
  Fix that the api error code is wrongly set.
diff: @@ -152,6 +152,7 @@ pub enum HttpError {
  BlobcacheMetrics(ApiError),
  BackendMetrics(ApiError),
  FsBackendInfo(ApiError),
  + InflightMetrics(ApiError),
  }
  fn success_response(body: Option<String>) -> Response {
  @@ -395,7 +396,7 @@ impl EndpointHandler for MetricsInflightHandler {
  match (req.method(), req.body.as_ref()) {
  (Method::Get, None) => {
  let r = kicker(ApiRequest::ExportInflightMetrics);
  - Ok(convert_to_response(r, HttpError::BlobcacheMetrics))
  + Ok(convert_to_response(r, HttpError::InflightMetrics))
  }
  _ => Err(HttpError::BadRequest),
  }
  