feat(update): verify etag updated

From [AIP-154](https://google.aip.dev/154) documentation:
```
The etag field must be provided by the server on output,
and values should conform to RFC 7232
```

Note: no validation is done that Etag conforms to `RFC 7232`.

README.md

@@ -158,6 +158,7 @@ Sample skips:
 | persisted            | The updated resource should be persisted and reachable with Get.                                            | Generated only if all are true: <ul><li>has Update method</li><li>Update method does not return long-running operation</li><li>has Get method</li></ul>                                                                                                 |
 | preserve create_time | The field create_time should be preserved when a '\*'-update mask is used.                                  | Generated only if all are true: <ul><li>has Update method</li><li>Update method does not return long-running operation</li><li>has field 'create_time'</li><li>resource has any required fields</li></ul>                                               |
 | etag mismatch        | Method should fail with Aborted if the supplied etag doesnt match the current etag value.                   | Generated only if all are true: <ul><li>has Update method</li><li>request has etag field</li><li>has field 'etag'</li></ul>                                                                                                                             |
+| etag updated         | Field etag should have a new value when the resource is successfully updated.                               | Generated only if all are true: <ul><li>has Update method</li><li>request has etag field</li><li>has field 'etag'</li></ul>                                                                                                                             |
 | not found            | Method should fail with NotFound if the resource does not exist.                                            | Generated only if all are true: <ul><li>has Update method</li></ul>                                                                                                                                                                                     |
 | invalid update mask  | The method should fail with InvalidArgument if the update_mask is invalid.                                  | Generated only if all are true: <ul><li>has Update method</li><li>Update method has update_mask</li></ul>                                                                                                                                               |
 | required fields      | Method should fail with InvalidArgument if any required field is missing when called with '\*' update_mask. | Generated only if all are true: <ul><li>has Update method</li><li>resource has any required fields</li></ul>                                                                                                                                            |

internal/aiptest/update/etag.go

@@ -39,3 +39,35 @@ var etagMismatch = suite.Test{
 		return nil
 	},
 }
+
+//nolint:gochecknoglobals
+var etagUpdated = suite.Test{
+	Name: "etag updated",
+	Doc: []string{
+		"Field etag should have a new value when the resource is successfully updated.",
+	},
+	OnlyIf: suite.OnlyIfs(
+		onlyif.HasMethod(aipreflect.MethodTypeUpdate),
+		onlyif.HasRequestEtag(aipreflect.MethodTypeUpdate),
+		onlyif.HasField("etag"),
+	),
+	Generate: func(f *protogen.GeneratedFile, scope suite.Scope) error {
+		if util.HasParent(scope.Resource) {
+			f.P("parent := ", ident.FixtureNextParent, "(t, false)")
+			f.P("created := fx.create(t, parent)")
+		} else {
+			f.P("created := fx.create(t)")
+		}
+		updateMethod, _ := util.StandardMethod(scope.Service, scope.Resource, aipreflect.MethodTypeUpdate)
+		util.MethodUpdate{
+			Resource: scope.Resource,
+			Method:   updateMethod,
+			Parent:   "parent",
+			Name:     "created.Name",
+			Etag:     "created.Etag",
+		}.Generate(f, "updated", "err", ":=")
+		f.P(ident.AssertNilError, "(t, err)")
+		f.P(ident.AssertCheck, "(t, updated.Etag != created.Etag)")
+		return nil
+	},
+}

internal/aiptest/update/update.go

@@ -19,6 +19,7 @@ var Suite = suite.Suite{
 		persisted,
 		preserveCreateTime,
 		etagMismatch,
+		etagUpdated,
 	},
 	TestGroups: []suite.TestGroup{
 		withResourceGroup,
@@ -41,6 +42,5 @@ var withResourceGroup = suite.TestGroup{
 		invalidUpdateMask,
 		requiredFields,
 		// TODO: add test for supplying wildcard as name
-		// TODO: add test for etags
 	},
 }