Allow to export data in gcov format

[cpaelzer]

Hi,
as pre-discussed with @brancz this is about allowing in addition to allowing data to be downloaded in pprof, we would allow data to be downloaded in the gcov format.

While it is true that experience shows that the performance gain tends to be limited when doing this with CPU sampled data, it might allow to do that at scale (vs higher overhead profiling).

To do so with GCC/LLVM toolchains as well the availability in gcov format would be helpful, as it can be used as input to their profile based optimization workflow.