benchmarks: Add subdirectory Benchmarks/ package (#119)

### Motivation

We'd like to be able to benchmark various parts of the runtime library
to make targeted and measurable performance improvements.

### Modifications

- Add a subdirectory directory package with the package-benchmark
plugin.
- Add a benchmark for `ISO8601DateTranscoder.encode(_:)`.
- Add temporary benchmarks of Foundation ISO8601 date encoding APIs to
guide improvements to `ISO8601DateTranscoder`.

### Result

- Can now run benchmarks using `swift package --package-path Benchmarks/
benchmark`.

### Test Plan

There is no CI for this currently. That can follow, but we wanted the
package plugin to unblock incoming updates to the date transcoder.

Author: simonjbeaumont

.gitignore

@@ -6,7 +6,7 @@ xcuserdata/
 DerivedData/
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
 .vscode
-/Package.resolved
+Package.resolved
 .ci/
 .docc-build/
 .swiftpm

.licenseignore

@@ -6,5 +6,5 @@
 .github/
 **.md
 **.txt
-Package.swift
+**Package.swift
 docker/*

.swiftformatignore

@@ -1 +1 @@
-Package.swift
+**Package.swift

Benchmarks/Benchmarks/OpenAPIRuntimeBenchmarks/Benchmarks.swift

@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2024 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+import Benchmark
+import OpenAPIRuntime
+import Foundation
+
+let benchmarks = {
+    let defaultMetrics: [BenchmarkMetric] = [.mallocCountTotal, .cpuTotal]
+
+    Benchmark(
+        "ISO8601DateTranscoder.encode(_:)",
+        configuration: Benchmark.Configuration(
+            metrics: defaultMetrics,
+            scalingFactor: .kilo,
+            maxDuration: .seconds(10_000_000),
+            maxIterations: 5
+        )
+    ) { benchmark in
+        let transcoder = ISO8601DateTranscoder()
+        benchmark.startMeasurement()
+        for _ in benchmark.scaledIterations { blackHole(try transcoder.encode(.distantFuture)) }
+    }
+
+    Benchmark(
+        "ISO8601DateFormatter.string(from:)",
+        configuration: Benchmark.Configuration(
+            metrics: defaultMetrics,
+            scalingFactor: .kilo,
+            maxDuration: .seconds(10_000_000),
+            maxIterations: 5
+        )
+    ) { benchmark in
+        let formatter = ISO8601DateFormatter()
+        benchmark.startMeasurement()
+        for _ in benchmark.scaledIterations { blackHole(formatter.string(from: .distantFuture)) }
+    }
+
+    Benchmark(
+        "Date.ISO8601Format(_:)",
+        configuration: Benchmark.Configuration(
+            metrics: defaultMetrics,
+            scalingFactor: .kilo,
+            maxDuration: .seconds(10_000_000),
+            maxIterations: 5
+        )
+    ) { benchmark in
+        benchmark.startMeasurement()
+        for _ in benchmark.scaledIterations { blackHole(Date.distantFuture.ISO8601Format()) }
+    }
+}

Benchmarks/Package.swift

@@ -0,0 +1,24 @@
+// swift-tools-version: 5.9
+import PackageDescription
+
+let package = Package(
+    name: "swift-openapi-runtime-benchmarks",
+    platforms: [ .macOS("14") ],
+    dependencies: [
+        .package(name: "swift-openapi-runtime", path: "../"),
+        .package(url: "https://github.com/ordo-one/package-benchmark.git", from: "1.22.0"),
+    ],
+    targets: [
+        .executableTarget(
+            name: "OpenAPIRuntimeBenchmarks",
+            dependencies: [
+                .product(name: "Benchmark", package: "package-benchmark"),
+                .product(name: "OpenAPIRuntime", package: "swift-openapi-runtime"),
+            ],
+            path: "Benchmarks/OpenAPIRuntimeBenchmarks",
+            plugins: [
+                .plugin(name: "BenchmarkPlugin", package: "package-benchmark")
+            ]
+        ),
+    ]
+)