[Feature Request] Add support for custom metrics

[mjameswh]

Context

This is the first half of the work required to reach #1177. With temporalio/sdk-core#544 landed, we need to:

• Update to latest Core SDK, making sure that existing Prometheus and OpenTelemetry options still work correctly;
• Add support for users to add/record metrics;
• Remove support for Tracing.

Extra context

API should mostly follow the precedent established by DotNET SDK and Python SDK.