Expose etcd client MaxCallSendMsgSize config

The etcdv3 client has a default request send limit of 2.0 MiB. This change
exposes the configuration option to increase that limit enabling larger
state using the etcdv3 backend.

This also requires that the corresponding --max-request-bytes flag be
increased on the server side. The default there is 1.5 MiB.

Fixes https://github.com/hashicorp/terraform/issues/25745

internal/backend/remote-state/etcdv3/backend.go

@@ -15,6 +15,7 @@ const (
 	usernameEnvVarName = "ETCDV3_USERNAME"
 	passwordKey        = "password"
 	passwordEnvVarName = "ETCDV3_PASSWORD"
+	maxRequestBytesKey = "max_request_bytes"
 	prefixKey          = "prefix"
 	lockKey            = "lock"
 	cacertPathKey      = "cacert_path"
@@ -49,6 +50,13 @@ func New() backend.Backend {
 				DefaultFunc: schema.EnvDefaultFunc(passwordEnvVarName, ""),
 			},
 
+			maxRequestBytesKey: &schema.Schema{
+				Type:        schema.TypeInt,
+				Optional:    true,
+				Description: "The max request size to send to etcd.",
+				Default:     0,
+			},
+
 			prefixKey: &schema.Schema{
 				Type:        schema.TypeString,
 				Optional:    true,
@@ -128,6 +136,9 @@ func (b *Backend) rawClient() (*etcdv3.Client, error) {
 	if v, ok := b.data.GetOk(passwordKey); ok && v.(string) != "" {
 		config.Password = v.(string)
 	}
+	if v, ok := b.data.GetOk(maxRequestBytesKey); ok && v.(int) != 0 {
+		config.MaxCallSendMsgSize = v.(int)
+	}
 	if v, ok := b.data.GetOk(cacertPathKey); ok && v.(string) != "" {
 		tlsInfo.TrustedCAFile = v.(string)
 	}