Change process.open_file_descriptors

[braydonk]

Area(s)

area:process

What's missing?

Note: first discussed in #1797

process.open_file_descriptors has 2 problems:

1. The open_ is arguably redundant, since a "file descriptor" is itself a representation of an open file (i.e. there's no such thing as counting closed_file_descriptors)
2. It can be confusing to Windows users, since on Windows you'd be counting Handles, a distinct windows exclusive concept, rather than file_descriptors.

Describe the solution you'd like

I suggest renaming the metric to process.file_descriptors, or process.unix.file_descriptors. At first glance I like unix being in the name, but this would be the first time we use that as a platform name so we'll need to think a bit about that before making that jump.

There will also be a metric called process.windows.handles in the future. Hopefully the presence of this metric will alleviate the platform confusion.

[braydonk]

Marking as GA Blocker since open_file_descriptors is an existing metric in use by instrumentation today.