Avoids varargs and the allocation overhead of creating a string array.

During application profiling found that some allocation can be avoided
if we explicitly invoke the checkArgument method without varargs

Allocation profiling via async-profiler:

```
--- 1451114625215846968 bytes (21.40%), 10371 samples
  [ 0] java.lang.String[]
  [ 1] com.netflix.servo.tag.BasicTag.checkNotEmpty
  [ 2] com.netflix.servo.tag.BasicTag.<init>
  [ 3] com.netflix.servo.tag.Tags.newTag
  [ 4] com.netflix.servo.monitor.MonitorConfig$Builder.withTag
```