Update stats format when read all values

[yixinglu]

Response format when exec $ curl -G 'http://localhost:5922/stats'

original format

value=0
name=num_query_errors.sum.5
value=0
name=num_query_errors.sum.60
value=0
name=num_query_errors.sum.600
...

new format

slow_query_latency_us.avg.5=0
num_slow_queries.rate.5=0
num_slow_queries.rate.60=0
num_slow_queries.sum.600=0
num_slow_queries.sum.3600=0
...