what do you guys user for collecting metrics? cloudwatch, datadog?

i'm looking for something that can operate at scale and supports some advanced features like arbitrary quantiles, extensible aggregation functions, extensible "combinator" type things to combine metrics to form new metrics etc

doesn't have to be hosted

i'm using datadog now, and one thing that's missing is boolean operators, like if metric.a == 0 then metric.b else metric.b / metric.a

I think logstash is a good solution