error[E0277]: `Label` doesn't implement `std::fmt::Display`
 --> tests/ui/labels/unimplemented_format.rs:4:20
  |
4 | #[metrics(format = "{}")]
  |                    ^^^^ `Label` cannot be formatted with the default formatter
  |
 ::: $RUST/core/src/macros/mod.rs
  |
  |         $dst.write_fmt($crate::format_args!($($arg)*))
  |                        ------------------------------ in this macro invocation
  |
  = help: the trait `std::fmt::Display` is not implemented for `Label`, which is required by `&Label: std::fmt::Display`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: this error originates in the macro `$crate::format_args` (in Nightly builds, run with -Z macro-backtrace for more info)