fix display unit

Author: avolokhov

Sources/Prometheus/MetricTypes/Summary.swift

@@ -139,7 +139,8 @@ public class PromSummary<NumType: DoubleRepresentable, Labels: SummaryLabels>: P
 
     // Updated for SwiftMetrics 2.0 to be unit agnostic if displayUnit is set or default to nanoseconds.
     private func format(_ v: Double) -> Double {
-        let displayUnitScale = self.displayUnit?.scaleFromNanoseconds ?? 1
+        let displayUnit = lock.withLock { self.displayUnit }
+        let displayUnitScale = displayUnit?.scaleFromNanoseconds ?? 1
         return v / Double(displayUnitScale)
     }
 

Tests/SwiftPrometheusTests/SummaryTests.swift

@@ -36,7 +36,8 @@ final class SummaryTests: XCTestCase {
 
     func testSummary() {
         let summary = Timer(label: "my_summary")
-        
+        summary.handler.preferDisplayUnit(.nanoseconds)
+
         summary.recordNanoseconds(1)
         summary.recordNanoseconds(2)
         summary.recordNanoseconds(4)