Self-recursive debug implementations

[tustvold]

I'm not sure how widespread this is, but if you try to debug print a prometheus sync instrument it leads to a stack overflow. Or if you debug print with {:#?} it effectively deadlocks trying to print an infinite string, I suspect you would get a stack overflow eventually if you left it for long enough.

The code snippet below demonstrates this problem, it overflows the stack

let exporter = opentelemetry_prometheus::exporter()
            .with_resource(Resource::new(vec![KeyValue::new("R", "V")]))
            .init();

let provider = exporter.provider().unwrap();
let meter = provider.meter("foo", None);
let counter = meter.f64_counter("test").init();
counter.add(23., &[KeyValue::new("foo", "bar")]);

println!("{:?}", counter);

[TommyCpp]

Looks like the reason is Instrument refers to Accumulator, which contains references to AsyncInstrument. And AsyncInstrument also contains a reference to Instrument, creating a loop.

[TommyCpp]

close via #552