feat(core): Decouple metrics aggregation from client

[timfish]

This is a re-opening of #10596 which was closed automatically after the branch is was based off was closed.

Part of the work for #10595

This PR decouples metrics aggregation from the client and stores the aggregator globally the first time it is required.

This PR does not remove captureAggregateMetrics from the BaseClient or make sendEnvelope public as this can occur in a later PR. This will remove the remaining metrics code from the most basic tree-shaken Sentry configuration.

The default (node) metrics implementation remains in @sentry/core because it's required for Deno and Vercel-Edge which don't ref. @sentry/node.

There's a load of extra deletions and cleanup that can occur for v8 when the integration is deleted. For example BrowserMetricsAggregator can probably move to @sentry/browser.

[timfish]

Rather than deprecate things that would be semver breaking changes for v8, I've just deleted them since we're now on the v8 path....

This means I've removed:

• BaseClient.metricsAggregator
• Sentry.metrics.MetricsAggregator
• Sentry.metrics.metricsAggregatorIntegration

[AbhiPrasad]

Would be nice to get some more integration tests for metrics in general so we can validate that this doesn't break anything.

Also now that develop is the v8 branch, feel free to make any breaking changes you think are needed to improve the API here.

Woops didn't read your comment. Can we add a migration doc please!

[timfish]

I've now added to MIGRATION.md and added a browser integration test for metrics!