Define reentrant safety of JavaScript Metrics API callbacks

[legendecas]

• This only affects the JavaScript OpenTelemetry library

As https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#asynchronous-counter-creation states:

OpenTelemetry API authors SHOULD define whether this callback function needs to be reentrant safe / thread safe or not.

We should define the reentrant safety of JavaScript Metrics API callbacks.

[legendecas]

The spec has updated to define that:

Callback functions MUST be documented as follows for the end user:

• Callback functions SHOULD be reentrant safe. The SDK expects to evaluate callbacks for each MetricReader independently.
• Callback functions SHOULD NOT take an indefinite amount of time.
• Callback functions SHOULD NOT make duplicate observations (more than one Measurement with the same attributes) across all registered callbacks.

We should follow the latest definition.