NUTCH-3043 Generator: count URLs rejected by URL filters

[sebastian-nagel]

• add counters URL_FILTERS_REJECTED and URL_FILTER_EXCEPTION
• simplify logging statement
• remove unnecessary cast

[sebastian-nagel]

Hi @lewismc:

• "use parameterized logging": done
• "augment the metrics documentation once this is merged.": will do
• "we could also create a test for the counters.": for now, TestGenerator is not based on MRUNIT. The various Generator::generate(...) return the number of generated segments without a way to access the counters (they're logged, however). I'd prefer to track this in a separate issue, because it would require to many code changes to read the counters.

[lewismc]

Excellent @sebastian-nagel 👍 I agree

[sebastian-nagel]

Thanks, @lewismc! The metrics wiki page was updated.