Summary:
Please consider this small PR providing access to the `MemoryUsage::GetApproximateMemoryUsageByType` function in plain C API. Actually I'm working on Go application and now trying to investigate the reasons of high memory consumption (#4313). Go [wrappers](https://github.com/tecbot/gorocksdb) are built on the top of Rocksdb C API. According to the #706, `MemoryUsage::GetApproximateMemoryUsageByType` is considered as the best option to get database internal memory usage stats, but it wasn't supported in C API yet.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4340

Differential Revision: D9655135

Pulled By: ajkr

fbshipit-source-id: a3d2f3f47c143ae75862fbcca2f571ea1b49e14a