Add internal/params pacakge test

docs/contributing/coding-style.md

@@ -264,6 +264,30 @@ func (s *something) SetSignedTok(st string) {
 }
 ```
 
+### Unused Variables
+
+An unused variable may increase the complexity of the source code, it may confuse the developer hence introduce a new bug.
+So please delete the unused variable.
+
+Generally, the unused variable should be reported during compilation, but in some cases, the compiler may not report an error. This is an example of the unused variable declaration that does not cause a compilation error.<br />
+
+```go
+type server struct {
+    addr string
+    port int
+}
+
+srv := &server {
+    addr: "192.168.33.10:1234",
+    // port <- unused variables
+}
+
+if err := srv.Run(); err != nil {
+    log.Fatal(err)
+}
+
+```
+
 ### Error handling
 
 All errors should define in [internal/errors package](https://github.com/vdaas/vald/blob/master/internal/errors). All errors should be start with `Err` prefix, and all errors should be handle if possible.
@@ -603,3 +627,44 @@ We do not suggest to modify the generated code other than the `tests` variable,
         }
         // generated test code
     ```
+
+1. Unused fields
+
+    By default, the template provides `fields` structure to initialize object of the test target. 
+
+    But in some cases, not all `fields` are needed, so please delete the unnecessary fields.
+
+    For example, the following struct and the corresponding function:
+
+    ```golang
+    type server struct {
+        addr string
+        port int
+    }
+    func (s *server) Addr() string {
+        return s.addr
+    }
+    ```
+
+    And the generated test code is:
+    ```golang
+    func Test_server_Addr(t *testing.T) {
+        type fields struct {
+            addr string
+            port int
+        }
+        type want struct {
+            // generated test code
+    ```
+
+    Since the `port` variable is not used in this test case, you can delete the `port` definition in the test case.
+
+    ```golang
+    func Test_server_Addr(t *testing.T) {
+        type fields struct {
+            addr string
+            // port int   <-- this line should be deleted
+        }
+        type want struct {
+            // generated test code
+    ```

internal/params/option.go

@@ -30,36 +30,42 @@ var (
 	}
 )
 
+// WithConfigFilePathKeys returns Option that sets filePath.keys.
 func WithConfigFilePathKeys(keys ...string) Option {
 	return func(p *parser) {
 		p.filePath.keys = append(p.filePath.keys, keys...)
 	}
 }
 
+// WithConfigFilePathDefault returns Option that sets filePath.defaultPath.
 func WithConfigFilePathDefault(path string) Option {
 	return func(p *parser) {
 		p.filePath.defaultPath = path
 	}
 }
 
+// WithConfigFileDescription returns Option that sets filePath.description.
 func WithConfigFileDescription(desc string) Option {
 	return func(p *parser) {
 		p.filePath.description = desc
 	}
 }
 
+// WithVersionKeys returns Option that sets version.keys.
 func WithVersionKeys(keys ...string) Option {
 	return func(p *parser) {
 		p.version.keys = append(p.version.keys, keys...)
 	}
 }
 
+// WithVersionFlagDefault returns Option that sets version.defaultFlag.
 func WithVersionFlagDefault(flag bool) Option {
 	return func(p *parser) {
 		p.version.defaultFlag = flag
 	}
 }
 
+// WithVersionDescription returns Option that sets version.description.
 func WithVersionDescription(desc string) Option {
 	return func(p *parser) {
 		p.version.description = desc