Fix Sendability warnings

Author: gjcairo

Sources/CoreMetrics/Locks.swift

@@ -46,7 +46,7 @@ import Musl
 /// of lock is safe to use with `libpthread`-based threading models, such as the
 /// one used by NIO. On Windows, the lock is based on the substantially similar
 /// `SRWLOCK` type.
-internal final class Lock {
+internal final class Lock: Sendable {
     #if os(Windows)
     fileprivate let mutex: UnsafeMutablePointer<SRWLOCK> =
         UnsafeMutablePointer.allocate(capacity: 1)

Sources/CoreMetrics/Metrics.swift

@@ -797,10 +797,11 @@ internal final class AccumulatingRoundingFloatingPointCounter: FloatingPointCoun
 }
 
 /// Wraps a RecorderHandler, adding support for incrementing values by storing an accumulated  value and recording increments to the underlying CounterHandler after crossing integer boundaries.
-internal final class AccumulatingMeter: MeterHandler {
+/// - Note: we can annotate this class as `@unchecked Sendable` because we are manually gating access to mutable state (i.e., the `value` property) via a Lock.
+internal final class AccumulatingMeter: MeterHandler, @unchecked Sendable {
     private let recorderHandler: RecorderHandler
-    // FIXME: use atomics when available
-    private var value: Double = 0
+    // FIXME: use swift-atomics when floating point support is available
+    private var value: Double =  0
     private let lock = Lock()
 
     init(label: String, dimensions: [(String, String)]) {

Sources/Metrics/Metrics.swift

@@ -71,6 +71,8 @@ extension Timer {
             self.recordSeconds(value)
         case .never:
             self.record(0)
+        @unknown default:
+            self.record(0)
         }
     }
 }

Tests/MetricsTests/MetricsTests.swift

@@ -196,6 +196,8 @@ extension DispatchTimeInterval {
             return value * 1_000_000_000
         case .never:
             return 0
+        @unknown default:
+            return 0
         }
     }
 }