feat(metrics): report scheduler utilization in runtime metrics

[solnic]

Adds elixir.runtime.scheduler.utilization, a number between 0.0 and 1.0 for how much of the VM's capacity to run code is actually in use. 0.0 is idle, 1.0 means there is no headroom left.

In practical terms, this will show you how busy the VM's main worker threads were - running application code plus the runtime's own work (GC, memory management, drivers, BIFs) - excluding time blocked on I/O.

Screenshot

If this is close to 1.0 very often for significant periods, you may want an alert set up.

[sentry]

Bug: The warning for missing :telemetry_poller is misleading. It only mentions elixir.runtime.scheduler.utilization won't be reported, but in fact all runtime metrics will be unavailable.
_{Severity: LOW}

Suggested Fix

Update the warning message in maybe_runtime_metrics_poller to clarify that if :telemetry_poller is not available, all runtime metrics will be disabled, not just elixir.runtime.scheduler.utilization.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: lib/sentry/application.ex#L129-L133

Potential issue: When runtime metrics are enabled but the `:telemetry_poller` dependency
is not available, a warning is logged. This warning is misleading because it only states
that `elixir.runtime.scheduler.utilization` will not be reported. In reality, the
absence of `:telemetry_poller` means that *no* runtime metrics will be collected,
including memory, run queue, and system count metrics. This can cause confusion for
developers trying to debug why they are not seeing any runtime metrics.