Make Lightstep metrics SDK exclusive until the new community SDK is available

[jmacd]

Is your feature request related to a problem? Please describe.
The new Lightstep metrics SDK is now better tested and has fewer known problems, relative to the current community SDK. Until the alpha community SDK is available, this library should switch to the Lightstep metrics SDK default.

Describe the solution you'd like
Change the default; let the user use WithLightstepMetricsSDK(false).

Describe alternatives you've considered
Removing the community SDK; this will create more work later, since it will inevitably be re-introduced.