Add shutdown method to OpenTelemetrySdk

[jack-berg]

To shutdown OpenTelemetry you have to do something like:

    // Configure sdk
    OpenTelemetrySdk sdk = OpenTelemetrySdk.builder()
        ...
        .build();
    // .. run the app
    
    // Shutdown sdk
    sdk.getSdkLoggerProvider().shutdown().join(10, TimeUnit.SECONDS);
    sdk.getSdkTracerProvider().shutdown().join(10, TimeUnit.SECONDS);
    sdk.getSdkMeterProvider().shutdown().join(10, TimeUnit.SECONDS);

It's weird that there isn't a single OpenTelemetrySdk.shutodown() method that calls shutdown() on each of the providers and aggregates the responses.

Not a huge deal, but works against the idea that the SDK is a single thing you configure and instrument an application with. Currently, OpenTelemetrySdk acts as more of a collection of discreet tools.