Allow async instrument to close via observable

Sources/OpenTelemetryApi/Metrics/Stable/DefaultStableMeter.swift

@@ -66,9 +66,13 @@ public class DefaultStableMeter : StableMeter {
     }
   }
 
-  private struct NoopObservableLongGauge : ObservableLongGauge {}
+  private struct NoopObservableLongGauge : ObservableLongGauge {
+      func close() {}
+  }
 
-  private struct NoopObservableDoubleGauge : ObservableDoubleGauge {}
+  private struct NoopObservableDoubleGauge : ObservableDoubleGauge {
+      func close() {}
+  }
 
   private class NoopDoubleUpDownCounterBuilder : DoubleUpDownCounterBuilder {
     func build() -> DoubleUpDownCounter {
@@ -80,7 +84,9 @@ public class DefaultStableMeter : StableMeter {
     }
   }
 
-  private struct NoopObservableDoubleUpDownCounter : ObservableDoubleUpDownCounter {}
+  private struct NoopObservableDoubleUpDownCounter : ObservableDoubleUpDownCounter {
+      func close() {}
+  }
 
   private struct NoopDoubleUpDownCounter : DoubleUpDownCounter {
     mutating func add(value: Double) {}
@@ -103,7 +109,9 @@ public class DefaultStableMeter : StableMeter {
     mutating func record(value: Int, attributes: [String : AttributeValue]) {}
   }
 
-  private class NoopObservableLongUpDownCounter : ObservableLongUpDownCounter {}
+  private class NoopObservableLongUpDownCounter : ObservableLongUpDownCounter {
+      func close() {}
+  }
 
   private class NoopDoubleHistogram : DoubleHistogram {
     func record(value: Double) {}
@@ -139,8 +147,13 @@ public class DefaultStableMeter : StableMeter {
     }
   }
 
-  private class NoopObservableLongCounter : ObservableLongCounter {}
-  private class NoopObservableDoubleCounter: ObservableDoubleCounter {}
+  private class NoopObservableLongCounter : ObservableLongCounter {
+      func close() {}
+  }
+
+  private class NoopObservableDoubleCounter: ObservableDoubleCounter {
+      func close() {}
+  }
 
   private class StableNoopDoubleCounterBuilder : DoubleCounterBuilder {
     func build() -> DoubleCounter {

Sources/OpenTelemetryApi/Metrics/Stable/ObservableDoubleCounter.swift

@@ -6,4 +6,5 @@
 import Foundation
 
 public protocol ObservableDoubleCounter {
+    func close()
 }

Sources/OpenTelemetryApi/Metrics/Stable/ObservableDoubleGauge.swift

@@ -6,4 +6,5 @@
 import Foundation
 
 public protocol ObservableDoubleGauge {
+    func close()
 }

Sources/OpenTelemetryApi/Metrics/Stable/ObservableDoubleUpDownCounter.swift

@@ -6,4 +6,5 @@
 import Foundation
 
 public protocol ObservableDoubleUpDownCounter {
+    func close()
 }

Sources/OpenTelemetryApi/Metrics/Stable/ObservableLongCounter.swift

@@ -6,4 +6,5 @@
 import Foundation
 
 public protocol ObservableLongCounter {
+    func close()
 }

Sources/OpenTelemetryApi/Metrics/Stable/ObservableLongGauge.swift

@@ -6,4 +6,5 @@
 import Foundation
 
 public protocol ObservableLongGauge {
+    func close()
 }

Sources/OpenTelemetryApi/Metrics/Stable/ObservableLongUpDownCounter.swift

@@ -6,4 +6,5 @@
 import Foundation
 
 public protocol ObservableLongUpDownCounter {
+    func close()
 }

Sources/OpenTelemetrySdk/Metrics/Stable/ObservableInstrumentSdk.swift

@@ -16,4 +16,7 @@ public struct ObservableInstrumentSdk : ObservableDoubleCounter, ObservableLongC
     }
 
     // todo: Java implementation uses closeables to remove this from the meterSharedState.callback Registation. investigate alternative?
+    public func close() {
+        meterSharedState.removeCallback(callback: callbackRegistration)
+    }
 }