Add flush method and make it implementation detail on how that happens

[ryzngard]

rzls imports telemetry by creating an ExportProvider with the path to devkit telemetry if it's installed and enabled. Unfortunately that means that dispose is being prematurely called when that provider is torn down. To help with this two things are done:

1. Make a flush method on ITelemetryReporter
2. Make when flush happens an implementation detail on the specific providers.

In this case, rzls will flush after the language server host exits. In other cases IDisposable will be used to flush telemetry (VS and OOP)

[davidwengier]

It's not clear to my why this couldn't just have been done by putting a using on line 80 of Program.cs, and have the Dispose method call Flush, but I have no problem with doing it explicitly either

[ryzngard]

It's not clear to my why this couldn't just have been done by putting a using on line 80 of Program.cs, and have the Dispose method call Flush, but I have no problem with doing it explicitly either

If we do that we double dispose, which isn't necessarily bad but also hard to spot and could lead to future bugs.

[davidwengier]

Oh okay, so we're essentially not in charge of dispose, so we're probably calling flush on the already disposed object, to work around that?

[ryzngard]

Yea. If we make it IDisposable then the container we use to discover the export from the devkit disposes of it

razor/src/Razor/src/rzls/Program.cs

Line 122 in e9199b6

using var exportProvider = await ExportProviderBuilder

It was causing me a headache to figure out why dispose was at startup and not when I would expect.