Add toString() to MapperConfig (prometheus#448)

Signed-off-by: Tom Wieczorek <t.wieczorek@yieldlab.de>

Author: twz123

simpleclient_dropwizard/src/main/java/io/prometheus/client/dropwizard/samplebuilder/MapperConfig.java

@@ -86,6 +86,11 @@ public MapperConfig(final String match, final String name, final Map<String, Str
         this.labels = labels;
     }
 
+    @Override
+    public String toString() {
+        return String.format("MapperConfig{match=%s, name=%s, labels=%s}", match, name, labels);
+    }
+
     public String getMatch() {
         return match;
     }

simpleclient_dropwizard/src/test/java/io/prometheus/client/dropwizard/samplebuilder/MapperConfigTest.java

@@ -3,6 +3,7 @@
 import io.prometheus.client.dropwizard.samplebuilder.MapperConfig;
 import org.junit.Test;
 
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -39,4 +40,19 @@ public void setLabels_WHEN_ExpressionDoesnNotMatchPattern_ThrowException() {
         labels.put("not valid", "${0}");
         mapperConfig.setLabels(labels);
     }
-}
+
+    @Test
+    public void toString_WHEN_EmptyConfig_AllGood() {
+        final MapperConfig mapperConfig = new MapperConfig();
+        assertEquals("MapperConfig{match=null, name=null, labels={}}", mapperConfig.toString());
+    }
+
+    @Test
+    public void toString_WHEN_FullyConfigured_AllGood() {
+        final MapperConfig mapperConfig = new MapperConfig();
+        mapperConfig.setMatch("com.company.meter.*.foo");
+        mapperConfig.setName("foo");
+        mapperConfig.setLabels(Collections.singletonMap("type", "${0}"));
+        assertEquals("MapperConfig{match=com.company.meter.*.foo, name=foo, labels={type=${0}}}", mapperConfig.toString());
+    }
+}