dubbo-samples-nacos introduces slf4j dependency (#299)

diff --git a/dubbo-samples-nacos/dubbo-samples-nacos-conditionrouter/pom.xml b/dubbo-samples-nacos/dubbo-samples-nacos-conditionrouter/pom.xml
index 62e6793..d74b906 100644
--- a/dubbo-samples-nacos/dubbo-samples-nacos-conditionrouter/pom.xml
+++ b/dubbo-samples-nacos/dubbo-samples-nacos-conditionrouter/pom.xml
@@ -34,6 +34,7 @@
         <dubbo.version>2.7.7</dubbo.version>
         <spring.version>4.3.16.RELEASE</spring.version>
         <junit.version>4.12</junit.version>
+        <slf4j.version>1.7.25</slf4j.version>
         <maven-compiler-plugin.version>3.7.0</maven-compiler-plugin.version>
     </properties>
 
@@ -67,6 +68,12 @@
                 <artifactId>junit</artifactId>
                 <version>${junit.version}</version>
             </dependency>
+
+            <dependency>
+                <groupId>org.slf4j</groupId>
+                <artifactId>slf4j-api</artifactId>
+                <version>${slf4j.version}</version>
+            </dependency>
         </dependencies>
     </dependencyManagement>
 
diff --git a/dubbo-samples-nacos/dubbo-samples-nacos-configcenter/pom.xml b/dubbo-samples-nacos/dubbo-samples-nacos-configcenter/pom.xml
index 466d720..79bde1a 100644
--- a/dubbo-samples-nacos/dubbo-samples-nacos-configcenter/pom.xml
+++ b/dubbo-samples-nacos/dubbo-samples-nacos-configcenter/pom.xml
@@ -34,6 +34,7 @@
         <dubbo.version>2.7.7</dubbo.version>
         <spring.version>4.3.16.RELEASE</spring.version>
         <junit.version>4.12</junit.version>
+        <slf4j.version>1.7.25</slf4j.version>
         <maven-compiler-plugin.version>3.7.0</maven-compiler-plugin.version>
     </properties>
 
@@ -67,6 +68,12 @@
                 <artifactId>junit</artifactId>
                 <version>${junit.version}</version>
             </dependency>
+
+            <dependency>
+                <groupId>org.slf4j</groupId>
+                <artifactId>slf4j-api</artifactId>
+                <version>${slf4j.version}</version>
+            </dependency>
         </dependencies>
     </dependencyManagement>
 
diff --git a/dubbo-samples-nacos/dubbo-samples-nacos-override/pom.xml b/dubbo-samples-nacos/dubbo-samples-nacos-override/pom.xml
index b83a0a0..7b0bc5f 100644
--- a/dubbo-samples-nacos/dubbo-samples-nacos-override/pom.xml
+++ b/dubbo-samples-nacos/dubbo-samples-nacos-override/pom.xml
@@ -34,6 +34,7 @@
         <dubbo.version>2.7.7</dubbo.version>
         <spring.version>4.3.16.RELEASE</spring.version>
         <junit.version>4.12</junit.version>
+        <slf4j.version>1.7.25</slf4j.version>
         <maven-compiler-plugin.version>3.7.0</maven-compiler-plugin.version>
     </properties>
 
@@ -67,6 +68,12 @@
                 <artifactId>junit</artifactId>
                 <version>${junit.version}</version>
             </dependency>
+
+            <dependency>
+                <groupId>org.slf4j</groupId>
+                <artifactId>slf4j-api</artifactId>
+                <version>${slf4j.version}</version>
+            </dependency>
         </dependencies>
     </dependencyManagement>
 
diff --git a/dubbo-samples-nacos/dubbo-samples-nacos-registry/pom.xml b/dubbo-samples-nacos/dubbo-samples-nacos-registry/pom.xml
index 03ce8b8..ac8affb 100644
--- a/dubbo-samples-nacos/dubbo-samples-nacos-registry/pom.xml
+++ b/dubbo-samples-nacos/dubbo-samples-nacos-registry/pom.xml
@@ -34,6 +34,7 @@
         <dubbo.version>2.7.7</dubbo.version>
         <spring.version>4.3.16.RELEASE</spring.version>
         <junit.version>4.12</junit.version>
+        <slf4j.version>1.7.25</slf4j.version>
         <maven-compiler-plugin.version>3.7.0</maven-compiler-plugin.version>
     </properties>
 
@@ -67,6 +68,12 @@
                 <artifactId>junit</artifactId>
                 <version>${junit.version}</version>
             </dependency>
+
+            <dependency>
+                <groupId>org.slf4j</groupId>
+                <artifactId>slf4j-api</artifactId>
+                <version>${slf4j.version}</version>
+            </dependency>
         </dependencies>
     </dependencyManagement>
 
diff --git a/dubbo-samples-nacos/dubbo-samples-nacos-tagrouter/pom.xml b/dubbo-samples-nacos/dubbo-samples-nacos-tagrouter/pom.xml
index 916aa1f..5d2cfe6 100644
--- a/dubbo-samples-nacos/dubbo-samples-nacos-tagrouter/pom.xml
+++ b/dubbo-samples-nacos/dubbo-samples-nacos-tagrouter/pom.xml
@@ -33,6 +33,7 @@
         <dubbo.version>2.7.7</dubbo.version>
         <spring.version>4.3.16.RELEASE</spring.version>
         <junit.version>4.12</junit.version>
+        <slf4j.version>1.7.25</slf4j.version>
         <maven-compiler-plugin.version>3.7.0</maven-compiler-plugin.version>
     </properties>
 
@@ -66,6 +67,12 @@
                 <artifactId>junit</artifactId>
                 <version>${junit.version}</version>
             </dependency>
+
+            <dependency>
+                <groupId>org.slf4j</groupId>
+                <artifactId>slf4j-api</artifactId>
+                <version>${slf4j.version}</version>
+            </dependency>
         </dependencies>
     </dependencyManagement>