reflection: add with_service_name method (#1042)

* reflection: add with_service_name method

Fixes: #682

* Update tonic-reflection/src/server.rs

Co-authored-by: Lucio Franco <luciofranco14@gmail.com>

tonic-reflection/src/server.rs

@@ -53,6 +53,7 @@ pub struct Builder<'b> {
     include_reflection_service: bool,
 
     service_names: Vec<String>,
+    use_all_service_names: bool,
     symbols: HashMap<String, Arc<FileDescriptorProto>>,
 }
 
@@ -65,6 +66,7 @@ impl<'b> Builder<'b> {
             include_reflection_service: true,
 
             service_names: Vec::new(),
+            use_all_service_names: true,
             symbols: HashMap::new(),
         }
     }
@@ -94,6 +96,16 @@ impl<'b> Builder<'b> {
         self
     }
 
+    /// Advertise a fully-qualified gRPC service name.
+    ///
+    /// If not called, then all services present in the registered file descriptor sets
+    /// will be advertised.
+    pub fn with_service_name(mut self, name: impl Into<String>) -> Self {
+        self.use_all_service_names = false;
+        self.service_names.push(name.into());
+        self
+    }
+
     /// Build a gRPC Reflection Service to be served via Tonic.
     pub fn build(mut self) -> Result<ServerReflectionServer<impl ServerReflection>, Error> {
         if self.include_reflection_service {
@@ -156,7 +168,9 @@ impl<'b> Builder<'b> {
 
         for service in &fd.service {
             let service_name = extract_name(prefix, "service", service.name.as_ref())?;
-            self.service_names.push(service_name.clone());
+            if self.use_all_service_names {
+                self.service_names.push(service_name.clone());
+            }
             self.symbols.insert(service_name.clone(), fd.clone());
 
             for method in &service.method {