Allow TaggedNameBuilder to overwrite previous value for a key

[xiaochuanyu]

Is there a particular reason why we allow duplicate key/val for tags?
I see this in the code:

metrics-datadog/metrics-datadog/src/main/java/org/coursera/metrics/datadog/TaggedName.java

Lines 96 to 100 in c497e68

	public TaggedNameBuilder addTag(String key, String val) {
	assertNonEmpty(key, "tagKey");
	encodedTags.add(new StringBuilder(key).append(':').append(val).toString());
	return this;
	}

This makes it not possible to overwrite tag values using the builder such as when you want to start with some default values for certain tags but overwrite them later conditionally.
The simplest example:

TaggedName.TaggedNameBuilder taggedNameBuilder =new TaggedName.TaggedNameBuilder().metricName("request_rate");
taggedNameBuilder.addTag("host_type", "default");
if (isSpecialCondition()) {
    taggedNameBuilder.addTag("host_type", "special");    
}
// etc

To maintain 100% backward compatibility, we can add a setTag(key, value) method that will overwrite or create tag with key/value.