Changes to telemetry with regards to discovery of python environments

[DonJayamanne]

sendTelemetryWhenDone(EventName.PYTHON_INTERPRETER_DISCOVERY, promise, undefined, { locator: this.name });

Change above code to capture number of items discovered.
Currently it just indicates the discovery ran to completion without errors, that's it, pretty much useless

Easy change.

[DonJayamanne]

Prescribed solution:

• Modify getInterpreters method of src/client/interpreter/locators/services/cacheableLocatorService.ts
• Send the counter (number of interpreters)
• Update InterpreterDiscovery type in src/client/telemetry/types.ts
  to account for this new property sent as part of the telemetry