feat: Add support for performance metrics

[timfish]

Closes #803

To enable metrics, you follow the same instructions as for node in the main process:

import * as Sentry from '@sentry/electron/main';

Sentry.init({
  dsn: '__DSN__',
  _experiments: {
    metricsAggregator: true,
  },
});

Sentry.metrics.increment('hits', 6);

In the renderer, no metric specific config is required:

import * as Sentry from '@sentry/electron/renderer';

Sentry.init();

Sentry.metrics.increment('hits', 6);

[AbhiPrasad]

Let's avoid having the user configure a MetricsAggregator for the renderer. We did this for treeshaking purposes in browser, but I think we can get away from that here.

The renderer can just configure it's metrics aggregator on init.

[timfish]

The renderer can just configure it's metrics aggregator on init.

Currently the main/renderer inits just call their respective node/browser inits with options. We'd need an Electron client to set aggregator.

It's probably more flexible to just add the MetricsAggregator integration to the default renderer integrations. This would also mean that if users copy the browser instructions and use new Sentry.metrics.MetricsAggregator(), one integration will get dropped and they don't end up using the browser implementation by mistake.