If FoundationEssentials is available use it (#158)

Use FoundationEssentials if it is available

### Motivation:

Reduce size of any application using swift-metrics on linux

---------

Co-authored-by: Konrad `ktoso` Malawski <konrad_malawski@apple.com>
Co-authored-by: Konrad `ktoso` Malawski <konrad.malawski@project13.pl>

Author: adam-fowler

Sources/Metrics/Metrics.swift

@@ -16,10 +16,14 @@
 // https://github.com/swiftlang/swift/issues/79285
 
 @_exported import CoreMetrics
-import Foundation
-
 @_exported import class CoreMetrics.Timer
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
+import Foundation
+#endif
+
 #if canImport(Dispatch)
 import Dispatch