Add internal/params pacakge test

docs/contributing/coding-style.md

@@ -264,6 +264,32 @@ func (s *something) SetSignedTok(st string) {
 }
 ```
 
+### Unused Variables
+
+It is an error to declare a variable without using it.
+while a variable that is initialized but not used is at least a wasted computation and perhaps indicative of a larger bug.
+So please delete unused variables.
+
+The following is an example of an unused variables that does not cause an error.<br>
+Please delete any variables that you don't need, as they can create bugs.
+
+```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 +629,27 @@ We do not suggest to modify the generated code other than the `tests` variable,
         }
         // generated test code
     ```
+
+1. 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)
+    }
+
+    ```

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