Consider recording application-level call statistics

[Xylus]

Description

Querying for complex server maps with thousands of agents is slow. Although server map generation code has been improved a lot in 1.7.0, the core of the problem remains - querying for each individual agent's call statistics, especially over a long period of time, is too expensive.
Until now, this was unavoidable as the server map had to include agent level histograms; but with the changes introduced in #3178, it is now possible to ignore agent level call statistics altogether.
(Note agent-level call statistics is still needed to for the histograms, but since they are lazy loaded 1 application at a time, we can stop querying for them for the server map API)

In order to make this happen, application-level call statistics must now be recorded.
This change would theoretically allow great improvement in server map performance as instead of querying for each agent's call statistics for every application, we can simply query once per application.

[yjqg6666]

Awesome feature. We really really need it. Before this feature is realized, we can only export the span data and let the hive/kylin to calculate and query for multiple dimension, it's a huge waste.