Fix Metadata's description for prettier printing (#2185)

This PR changes `Metadata`'s description to include quotes around string
values.

From this:

```
[("some-key", some-value)]
```

to this:

```
[("some-key", "some-value")]
```

---------

Co-authored-by: George Barnett <gbarnett@apple.com>

Author: gjcairo

Sources/GRPCCore/Metadata.swift

@@ -494,7 +494,13 @@ extension Metadata.Value: ExpressibleByArrayLiteral {
 
 extension Metadata: CustomStringConvertible {
   public var description: String {
-    String(describing: self.map({ ($0.key, $0.value) }))
+    if self.isEmpty {
+      return "[:]"
+    } else {
+      let elements = self.map { "\(String(reflecting: $0.key)): \(String(reflecting: $0.value))" }
+        .joined(separator: ", ")
+      return "[\(elements)]"
+    }
   }
 }
 
@@ -508,3 +514,14 @@ extension Metadata.Value: CustomStringConvertible {
     }
   }
 }
+
+extension Metadata.Value: CustomDebugStringConvertible {
+  public var debugDescription: String {
+    switch self {
+    case .string(let stringValue):
+      return String(reflecting: stringValue)
+    case .binary(let binaryValue):
+      return String(reflecting: binaryValue)
+    }
+  }
+}

Tests/GRPCCoreTests/MetadataTests.swift

@@ -314,4 +314,34 @@ struct MetadataTests {
       )
     }
   }
+
+  @Suite("Description")
+  struct Description {
+    let metadata: Metadata = [
+      "key1": "value1",
+      "key2": "value2",
+      "key-bin": .binary([1, 2, 3]),
+    ]
+
+    @Test("Metadata")
+    func describeMetadata() async throws {
+      #expect("\(self.metadata)" == #"["key1": "value1", "key2": "value2", "key-bin": [1, 2, 3]]"#)
+    }
+
+    @Test("Metadata.Value")
+    func describeMetadataValue() async throws {
+      for (key, value) in self.metadata {
+        switch key {
+        case "key1":
+          #expect("\(value)" == "value1")
+        case "key2":
+          #expect("\(value)" == "value2")
+        case "key-bin":
+          #expect("\(value)" == "[1, 2, 3]")
+        default:
+          Issue.record("Should not have reached this point")
+        }
+      }
+    }
+  }
 }