Support for Mixpanel Feature Flags

The SDK can now be used to load and evaluate Mixpanel Feature Flags (see https://docs.mixpanel.com/docs/featureflags). It supports two modes of operation, remote evaluation and local evaluation.

Remote Evaluation

const mixpanel = Mixpanel.init(`<YOUR_TOKEN>`, {
  remote_flags_config: {
    // optional configuration
    request_timeout_in_seconds: 5,
  },
});

// or to enable remote evaluation with no custom configuration:
const mixpanel = Mixpanel.init(`<YOUR_TOKEN>`, {
  remote_flags_config: {},
});

// A network call is made to fetch the assigned variant for a user
const variantValue = await mixpanel.remote_flags.getVariantValue(`<YOUR_FLAG_KEY>`, `<FALLBACK_VARIANT>`, {
  distinct_id : `<USER DISTINCT ID>`,
});

Local Evaluation

const mixpanel = Mixpanel.init(`YOUR_PROJECT_TOKEN`, {
  local_flags_config: {
    enable_polling: true,
    polling_interval_in_seconds: 60,
  },
});

// or to enable local evaluation with no custom configuration:
const mixpanel = Mixpanel.init(`<YOUR_TOKEN>`, {
  local_flags_config: {},
});

// Polls the endpoint for flag definitions 
mixpanel.local_flags.startPollingForDefinitions();

// User is quickly assigned to a variant based on polled flag definitions
const variantValue = mixpanel.local_flags.getVariantValue(`<YOUR_FLAG_KEY>`, `<FALLBACK_VARIANT>`, {
  distinct_id : `<USER_DISTINCT_ID`,
});

Further Node.js-specific Feature Flags documentation is available at https://docs.mixpanel.com/docs/tracking-methods/sdks/nodejs/nodejs-flags.