VM as a metric solution, metrtics API need to add more feature

[ahfuzhang]

I want to use metrics API to replace prometheus API.
But when I read source code, I think add more features will be wonderful:

1. keep fast and low memory use, and support profile report.
2. user can limit the metric count (for a server, don't let ruin robustness)
3. user can limit the memory use of metric api
4. remove short time metrics which not change for hours
5. make own binary marshal format and binary protocol, low cpu use and low bindwith.

thanks.

[ahfuzhang]

and:
6. low GC pressure
7. support huge count and huge size metric data.

[cristaloleg]

Do you have a profile which shows that these points are bottleneck ?