Emit correct metadata from StreamLogHandler (apple#251)

Co-authored-by: Konrad `ktoso` Malawski <[email protected]>

Author: slashmo

Sources/Logging/Logging.swift

@@ -1297,7 +1297,7 @@ public struct StreamLogHandler: LogHandler {
             metadata.merge(explicit, uniquingKeysWith: { _, explicit in explicit })
         }
 
-        return explicit
+        return metadata
     }
 
     private func prettify(_ metadata: Logger.Metadata) -> String? {

Tests/LoggingTests/LoggingTest+XCTest.swift

@@ -56,6 +56,7 @@ extension LoggingTest {
             ("testStreamLogHandlerOutputFormat", testStreamLogHandlerOutputFormat),
             ("testStreamLogHandlerOutputFormatWithMetaData", testStreamLogHandlerOutputFormatWithMetaData),
             ("testStreamLogHandlerOutputFormatWithOrderedMetadata", testStreamLogHandlerOutputFormatWithOrderedMetadata),
+            ("testStreamLogHandlerWritesIncludeMetadataProviderMetadata", testStreamLogHandlerWritesIncludeMetadataProviderMetadata),
             ("testStdioOutputStreamWrite", testStdioOutputStreamWrite),
             ("testStdioOutputStreamFlush", testStdioOutputStreamFlush),
             ("testOverloadingError", testOverloadingError),

Tests/LoggingTests/LoggingTest.swift

@@ -824,6 +824,24 @@ class LoggingTest: XCTestCase {
         XCTAssert(interceptStream.strings[1].contains("a=a1 b=b1"), "LINES: \(interceptStream.strings[1])")
     }
 
+    func testStreamLogHandlerWritesIncludeMetadataProviderMetadata() {
+        let interceptStream = InterceptStream()
+        LoggingSystem.bootstrapInternal({ _, metadataProvider in
+            StreamLogHandler(label: "test", stream: interceptStream, metadataProvider: metadataProvider)
+        }, metadataProvider: .exampleMetadataProvider)
+        let log = Logger(label: "test")
+
+        let testString = "my message is better than yours"
+        log.critical("\(testString)")
+
+        let messageSucceeded = interceptStream.interceptedText?.trimmingCharacters(in: .whitespacesAndNewlines).hasSuffix(testString)
+
+        XCTAssertTrue(messageSucceeded ?? false)
+        XCTAssertEqual(interceptStream.strings.count, 1)
+        let message = interceptStream.strings.first!
+        XCTAssertTrue(message.contains("example=example-value"), "message must contain metadata, was: \(message)")
+    }
+
     func testStdioOutputStreamWrite() {
         self.withWriteReadFDsAndReadBuffer { writeFD, readFD, readBuffer in
             let logStream = StdioOutputStream(file: writeFD, flushMode: .always)