Add scalar event logging function

[jeffcarp]

Currently there are three jax.monitoring methods for recording events:

• record_event(event: str, **kwargs: str | int)
• record_event_duration_secs(event: str, duration: float, **kwargs: str | int)
• record_event_time_span(event: str, start_time: float, end_time: float, **kwargs: str | int)

This PR adds a more general method, record_scalar:

• record_scalar(event: str, value: float | int, **kwargs: str | int)

which enables JAX core and its ecosystem libraries to emit a wider more flexible set of metrics, for example FLOPS usage, gradient norm/clipping stats, quantization ranges, and more. We (@jshin1394 and I) are also working on a plugin that will make it easy to send jax.monitoring metrics to TensorBoard and other tools.