spectator Id - ignore tags with empty or null value

[venki-netflix]

Currently it looks like we dont log the counter if the value of a tag is either null or empty value. This behavior of quietly dropping the counter is not noticeable and can have missing counters.

It will be nice to ignore the tags that are empty and still log other tags.

with following style of coding its easy to have null or empty tag values.
id.withTag("tag1", tag1Value).withTag("tag2",tag2Value).withTag("tag3", tag3Value);
registry.counter(id).increment();

basically {"name":"foo", "app":"", "action:"abc"} will be same as {"name":"foo", "action:"abc"}