Add Google Cloud worker instances to a target pool

* Background: A managed instance group of workers is used in backend
services for global load balancing (HTTP/HTTPS Ingress) and output
for custom global load balancing use cases
* Add worker instances to a target pool load balancing TCP/UDP
applications (NodePort or proxied). Output as `worker_target_pool`
* Health check for workers with a healthy Ingress controller. Forward
rules (regional) to target pools don't support different external and
internal ports so choosing nodes with Ingress allows proxying as a
workaround
* A target pool is a logical grouping only. It doesn't add costs to
clusters or worker pools

google-cloud/container-linux/kubernetes/workers/outputs.tf

@@ -1,4 +1,13 @@
+# Outputs for global load balancing
+
 output "instance_group" {
-  description = "Full URL of the worker managed instance group"
+  description = "Worker managed instance group full URL"
   value       = "${google_compute_region_instance_group_manager.workers.instance_group}"
 }
+
+# Outputs for regional load balancing
+
+output "target_pool" {
+  description = "Worker target pool self link"
+  value       = "${google_compute_target_pool.workers.self_link}"
+}

google-cloud/container-linux/kubernetes/workers/target_pool.tf

@@ -0,0 +1,21 @@
+# Target pool for TCP/UDP load balancing
+resource "google_compute_target_pool" "workers" {
+  name             = "${var.name}-worker-pool"
+  session_affinity = "NONE"
+
+  health_checks = [
+    "${google_compute_http_health_check.workers.name}",
+  ]
+}
+
+# HTTP Health Check (for TCP/UDP load balancing)
+# Forward rules (regional) to target pools don't support different external
+# and internal ports. Health check for nodes with Ingress controllers that
+# may support proxying or otherwise satisfy the check.
+resource "google_compute_http_health_check" "workers" {
+  name        = "${var.name}-target-pool-health"
+  description = "Health check for the worker target pool"
+
+  port         = 10254
+  request_path = "/healthz"
+}

google-cloud/container-linux/kubernetes/workers/workers.tf

@@ -1,4 +1,4 @@
-# Regional managed instance group of workers
+# Managed instance group of workers
 resource "google_compute_region_instance_group_manager" "workers" {
   name        = "${var.name}-worker-group"
   description = "Compute instance group of ${var.name} workers"
@@ -8,7 +8,8 @@ resource "google_compute_region_instance_group_manager" "workers" {
   instance_template  = "${google_compute_instance_template.worker.self_link}"
   region             = "${var.region}"
 
-  target_size = "${var.count}"
+  target_size  = "${var.count}"
+  target_pools = ["${google_compute_target_pool.workers.self_link}"]
 
   named_port {
     name = "http"