Support Kotlin-gRPC client CoroutineStub (line#2669)

Motivation:

gRPC for Kotlin is actively developed and
[0.1.1](https://github.com/grpc/grpc-kotlin/releases/tag/v0.1.1) has been released last week.

Modifications:

- Allow GrpcClientFactory creating client from CoroutineStub
- Migrate Kotlin example to gRPC-Kotlin

Result:

- Partially fixes line#2662
- You can now run Armeria gRPC client with `gproto*` and gRPC-Kotlin CoroutinStub

Co-authored-by: Gary Lo <gary.lo@airtasker.com>

dependencies.yml

@@ -197,6 +197,8 @@ io.grpc:
     - io.netty:netty-handler-proxy
     - io.netty:netty-transport
     - io.netty:netty-tcnative-boringssl-static
+  grpc-kotlin-stub: { version: &GRPC_KOTLIN_VERSION '0.1.1' }
+  protoc-gen-grpc-kotlin: { version: *GRPC_KOTLIN_VERSION }
 
 io.micrometer:
   micrometer-core:
@@ -420,7 +422,7 @@ org.jctools:
     - from: org.jctools
       to: com.linecorp.armeria.internal.shaded.jctools
 
-# If you want to change `org.jetbrains.kotlin:kotlin-allopen` version, 
+# If you want to change `org.jetbrains.kotlin:kotlin-allopen` version,
 # you also need to change `org.jetbrains.kotlin.jvm` version in `build.gradle`.
 org.jetbrains.kotlin:
   kotlin-allopen: { version: &KOTLIN_VERSION '1.3.71' }

examples/grpc-kotlin/build.gradle.kts

@@ -13,10 +13,9 @@ buildscript {
 plugins {
     application
     id("org.jetbrains.kotlin.jvm")
+    id("org.jlleitschuh.gradle.ktlint") version "9.2.1"
 }
 
-apply(plugin = "org.jlleitschuh.gradle.ktlint")
-
 application {
     mainClassName = "example.armeria.grpc.kotlin.MainKt"
 }
@@ -29,8 +28,10 @@ dependencies {
     runtimeOnly("org.slf4j:slf4j-simple")
 
     implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
-    implementation("org.jetbrains.kotlin:kotlin-reflect")
-    implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
+    implementation(kotlin("reflect"))
+    implementation(kotlin("stdlib-jdk8"))
+    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core")
+    implementation("io.grpc:grpc-kotlin-stub")
 
     testImplementation("jakarta.annotation:jakarta.annotation-api")
     testImplementation("net.javacrumbs.json-unit:json-unit-fluent")
@@ -44,3 +45,25 @@ tasks.withType<KotlinCompile> {
         freeCompilerArgs = listOf("-Xjsr305=strict", "-java-parameters")
     }
 }
+
+// TODO(ikhoon): gRPC-Kotlin compiler does not run well in Windows. The generated stub files are added
+//               intentionally. Remove this once gRPC-Kotlin compiler supports Windows as well.
+kotlin {
+    sourceSets["main"].apply {
+        kotlin.srcDir("gen-src/main/kotlinGrpc")
+    }
+}
+
+ktlint {
+    filter {
+        exclude { element -> element.file.path.contains("gen-src") }
+    }
+}
+
+tasks.clean {
+    for (path in delete) {
+        if (path == project.ext["genSrcDir"]) {
+            delete.remove(path)
+        }
+    }
+}