FTS: expose stats as rates

External monitoring systems tend to consume stat values from a service and compare them against statically defined thresholds.  We have identified the following metrics that are interesting for external monitoring:

1. total_queries_error
2. total_queries_timeout
3. total_queries_slow

These metrics would be more useful if exposed as rates, e.g.

1. queries_error_1m_rate
2. queries_error, 5min_rate
3. queries_error_15min_rate