Improve Instrumentation typings

[cmidgley]

This PR updates the .d.ts for Instrumentation. It adds the map and name methods, removes the old numeric types for all the instrumentation (which were incorrect depending on the platform/configuration), and adds a new string literal union type with the names of the instruments for use with map. It also allows for number | undefined for the index to nested operations such as Instrumentation.get(Instrumentation.map('Network Bytes Read')) which otherwise would be a type error.

[phoddie]

Thanks for doing this. We'll get it merged.

The list of key strings is a little ugly to manage, since it can vary by host. I guess we can keep that list up-to-date. But it will be possible to pass the TypeScript checks and still have that key be unavailable on a particular host. Not the end of the world (and no easy solution).

[cmidgley]

Might some hosts offer something unique or in addition to this list, or is this list the union of all possible names? It would be a type error if a different name were tried. I only added the list because the prior version had a list of (incorrect) name mappings, so perhaps instead it should just be a string (and not a union of string literal)?

[phoddie]

The current list of strings / names is not strictly the union of all possible names, since hosts can change that. But, I think for current purposes the list is fine as-is. Hosts don't introduce new instrumentation values often. The benefit of having a compile-time validity check on the string literals seems more important.