fix: self-recursive debug implementations in metrics.

fixes open-telemetry#550. We choose to break the loop at instrument level because debug info on `Accumulator` can be found via other structs and descriptors seem to be more important for instruments.

opentelemetry/src/sdk/metrics/mod.rs

@@ -513,12 +513,20 @@ impl sdk_api::SyncBoundInstrumentCore for Record {
     }
 }
 
-#[derive(Debug)]
 struct Instrument {
     descriptor: Descriptor,
     meter: Accumulator,
 }
 
+impl std::fmt::Debug for Instrument {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Instrument")
+            .field("descriptor", &self.descriptor)
+            .field("meter", &"Accumulator")
+            .finish()
+    }
+}
+
 impl sdk_api::InstrumentCore for Instrument {
     fn descriptor(&self) -> &Descriptor {
         &self.descriptor
@@ -579,3 +587,24 @@ impl sdk_api::MeterCore for Accumulator {
         self.0.register_runner(runner)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::metrics::MeterProvider;
+    use crate::sdk::export::metrics::ExportKindSelector;
+    use crate::sdk::metrics::controllers::pull;
+    use crate::sdk::metrics::selectors::simple::Selector;
+
+    // Prevent the debug message to get into loop
+    #[test]
+    fn test_debug_message() {
+        let controller = pull(
+            Box::new(Selector::Exact),
+            Box::new(ExportKindSelector::Delta),
+        )
+        .build();
+        let meter = controller.provider().meter("test", None);
+        let counter = meter.f64_counter("test").init();
+        println!("{:?}, {:?}, {:?}", controller, meter, counter);
+    }
+}