api: Give instruments a toString() including their name

This makes it much easier when testing to understand what the
values/arguments are at various parts of the code.

api/src/main/java/io/grpc/PartialMetricInstrument.java

@@ -89,4 +89,9 @@ abstract class PartialMetricInstrument implements MetricInstrument {
   public boolean isEnableByDefault() {
     return enableByDefault;
   }
+
+  @Override
+  public String toString() {
+    return getClass().getName() + "(" + getName() + ")";
+  }
 }