Fixed Histogram Concurrency Issue

Issue:
In Histogram, some values were being dropped when the histogram is accessed concurrently by multiple threads.

Fix:
In getOrCreateHistogram, the code was copying the dictionary into a local variable, and checking it for the label without a lock. If not there, it would take the lock again to add it. But in line 224, after taking the lock again, it was rechecking the local variable, which of course would not have changed in the meantime. But the original dictionary could have changed, and that's the one we should check the second time.

Improved the concurrency test, as it was missing the bug by only checking the total count and sum without labels.

Author: Sam-Amin

Sources/Prometheus/MetricTypes/Histogram.swift

@@ -221,7 +221,7 @@ public class PromHistogram<NumType: DoubleRepresentable>: PromMetric {
             return histogram
         } else {
             return lock.withLock {
-                if let histogram = subHistograms[labels] {
+                if let histogram = self.subHistograms[labels] {
                     precondition(histogram.name == self.name,
                                  """
                                  Somehow got 2 subHistograms with the same data type  / labels 

Tests/SwiftPrometheusTests/HistogramTests.swift

@@ -29,7 +29,7 @@ final class HistogramTests: XCTestCase {
                                              helpText: "Histogram for testing",
                                              buckets: Buckets.exponential(start: 1, factor: 2, count: 63))
         let elg = MultiThreadedEventLoopGroup(numberOfThreads: 8)
-        let semaphore = DispatchSemaphore(value: 2)
+        let semaphore = DispatchSemaphore(value: 0)
         _ = elg.next().submit {
             for _ in 1...1_000 {
                 let labels = DimensionLabels([("myValue", "1")])
@@ -51,9 +51,18 @@ final class HistogramTests: XCTestCase {
             semaphore.signal()
         }
         semaphore.wait()
+        semaphore.wait()
         try elg.syncShutdownGracefully()
-        XCTAssertTrue(histogram.collect().contains("my_histogram_count 4000.0"))
-        XCTAssertTrue(histogram.collect().contains("my_histogram_sum 4000.0"))
+
+        let output = histogram.collect()
+        XCTAssertTrue(output.contains("my_histogram_count 4000.0"))
+        XCTAssertTrue(output.contains("my_histogram_sum 4000.0"))
+
+        XCTAssertTrue(output.contains(#"my_histogram_count{myValue="1"} 2000.0"#))
+        XCTAssertTrue(output.contains(#"my_histogram_sum{myValue="1"} 2000.0"#))
+
+        XCTAssertTrue(output.contains(#"my_histogram_count{myValue="2"} 2000.0"#))
+        XCTAssertTrue(output.contains(#"my_histogram_sum{myValue="2"} 2000.0"#))
     }
 
     func testHistogramSwiftMetrics() {