Add pubsub service to notification engine

docs/services/gcppubsub.md

@@ -0,0 +1,106 @@
+# GCP Pub/Sub
+
+## Parameters
+
+This notification service is capable of sending messages to Google Cloud Pub/Sub topics.
+
+* `project` - GCP project ID where the Pub/Sub topic is located.
+* `topic` - name of the Pub/Sub topic you are intending to send messages to. Can be overridden with target destination annotation.
+* `keyFile` - optional, path to GCP service account key file. If not provided, uses Application Default Credentials
+
+## Example
+
+### Using Service Account Key File
+
+Resource Annotation:
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+  annotations:
+    notifications.argoproj.io/subscribe.on-deployment-ready.gcppubsub: "override-topic"
+```
+
+ConfigMap:
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: argocd-notifications-cm
+data:
+  service.gcppubsub: |
+    project: "my-gcp-project"
+    topic: "my-topic"
+    keyFile: "/var/secrets/gcp/key.json"
+
+  template.deployment-ready: |
+    message: |
+      Deployment {{.obj.metadata.name}} is ready!
+    gcppubsub:
+      attributes:
+        app: "{{.obj.metadata.name}}"
+        environment: "production"
+
+  trigger.on-deployment-ready: |
+    - when: any(obj.status.conditions, {.type == 'Available' && .status == 'True'})
+      send: [deployment-ready]
+    - oncePer: obj.metadata.annotations["generation"]
+```
+
+### Using Workload Identity (GKE)
+
+When running on GKE with Workload Identity enabled, you can omit the `keyFile` parameter and the service will use the default credentials from the service account bound to the Kubernetes service account.
+
+Resource Annotation:
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+  annotations:
+    notifications.argoproj.io/subscribe.on-deployment-ready.gcppubsub: ""
+```
+
+ConfigMap:
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: argocd-notifications-cm
+data:
+  service.gcppubsub: |
+    project: "my-gcp-project"
+    topic: "my-topic"
+
+  template.deployment-ready: |
+    message: |
+      Deployment {{.obj.metadata.name}} is ready!
+    gcppubsub:
+      attributes:
+        app: "{{.obj.metadata.name}}"
+        status: "{{.obj.status.conditions[0].type}}"
+
+  trigger.on-deployment-ready: |
+    - when: any(obj.status.conditions, {.type == 'Available' && .status == 'True'})
+      send: [deployment-ready]
+    - oncePer: obj.metadata.annotations["generation"]
+```
+
+## Message Attributes
+
+Pub/Sub messages support custom attributes (key-value pairs) that can be used for filtering or routing. You can include attributes in your template:
+
+```yaml
+template.deployment-ready: |
+  message: |
+    Deployment {{.obj.metadata.name}} is ready!
+  gcppubsub:
+    attributes:
+      severity: "info"
+      app: "{{.obj.metadata.name}}"
+      namespace: "{{.obj.metadata.namespace}}"
+      timestamp: "{{(call .time.Now).Unix}}"
+```
+
+Attributes support template variables just like the message body, allowing you to dynamically populate metadata from the object being monitored.

go.mod

@@ -3,6 +3,7 @@ module github.com/argoproj/notifications-engine
 go 1.24.0
 
 require (
+	cloud.google.com/go/pubsub v1.49.0
 	github.com/Masterminds/sprig/v3 v3.3.0
 	github.com/OvyFlash/telegram-bot-api v0.0.0-20241219171906-3f2ca0c14ada
 	github.com/PagerDuty/go-pagerduty v1.8.0
@@ -33,7 +34,7 @@ require (
 	go.yaml.in/yaml/v3 v3.0.4
 	golang.org/x/time v0.11.0
 	gomodules.xyz/notify v0.1.1
-	google.golang.org/api v0.223.0
+	google.golang.org/api v0.227.0
 	k8s.io/api v0.34.0
 	k8s.io/apimachinery v0.34.0
 	k8s.io/client-go v0.34.0
@@ -42,9 +43,11 @@ require (
 )
 
 require (
+	cloud.google.com/go v0.120.0 // indirect
 	cloud.google.com/go/auth v0.15.0 // indirect
 	cloud.google.com/go/auth/oauth2adapt v0.2.7 // indirect
 	cloud.google.com/go/compute/metadata v0.6.0 // indirect
+	cloud.google.com/go/iam v1.4.2 // indirect
 	dario.cat/mergo v1.0.1 // indirect
 	github.com/Masterminds/goutils v1.1.1 // indirect
 	github.com/Masterminds/semver/v3 v3.3.0 // indirect
@@ -75,7 +78,7 @@ require (
 	github.com/google/go-querystring v1.1.0 // indirect
 	github.com/google/go-tpm v0.9.5 // indirect
 	github.com/google/s2a-go v0.1.9 // indirect
-	github.com/googleapis/enterprise-certificate-proxy v0.3.4 // indirect
+	github.com/googleapis/enterprise-certificate-proxy v0.3.6 // indirect
 	github.com/googleapis/gax-go/v2 v2.14.1 // indirect
 	github.com/gorilla/websocket v1.5.4-0.20250319132907-e064f32e3674 // indirect
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
@@ -102,22 +105,27 @@ require (
 	github.com/spf13/pflag v1.0.6 // indirect
 	github.com/stretchr/objx v0.5.2 // indirect
 	github.com/x448/float16 v0.8.4 // indirect
+	go.opencensus.io v0.24.0 // indirect
 	go.opentelemetry.io/auto/sdk v1.1.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.59.0 // indirect
 	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.59.0 // indirect
-	go.opentelemetry.io/otel v1.34.0 // indirect
-	go.opentelemetry.io/otel/metric v1.34.0 // indirect
-	go.opentelemetry.io/otel/trace v1.34.0 // indirect
+	go.opentelemetry.io/otel v1.35.0 // indirect
+	go.opentelemetry.io/otel/metric v1.35.0 // indirect
+	go.opentelemetry.io/otel/trace v1.35.0 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	golang.org/x/crypto v0.38.0 // indirect
 	golang.org/x/net v0.38.0 // indirect
-	golang.org/x/oauth2 v0.27.0 // indirect
+	golang.org/x/oauth2 v0.28.0 // indirect
+	golang.org/x/sync v0.14.0 // indirect
 	golang.org/x/sys v0.33.0 // indirect
 	golang.org/x/term v0.32.0 // indirect
 	golang.org/x/text v0.25.0 // indirect
 	gomodules.xyz/envconfig v1.3.1-0.20190308184047-426f31af0d45 // indirect
-	google.golang.org/genproto/googleapis/rpc v0.0.0-20250219182151-9fdb1cabc7b2 // indirect
-	google.golang.org/grpc v1.70.0 // indirect
-	google.golang.org/protobuf v1.36.5 // indirect
+	google.golang.org/genproto v0.0.0-20250303144028-a0af3efb3deb // indirect
+	google.golang.org/genproto/googleapis/api v0.0.0-20250313205543-e70fdf4c4cb4 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20250313205543-e70fdf4c4cb4 // indirect
+	google.golang.org/grpc v1.71.0 // indirect
+	google.golang.org/protobuf v1.36.6 // indirect
 	gopkg.in/alexcesaro/quotedprintable.v3 v3.0.0-20150716171945-2caba252f4dc // indirect
 	gopkg.in/evanphx/json-patch.v4 v4.12.0 // indirect
 	gopkg.in/gomail.v2 v2.0.0-20160411212932-81ebce5c23df // indirect