Function underPressureFastifyPlugin

underPressureFastifyPlugin(
appName: Readonly<Partial<MiddlewareOptions>>,
): FastifyPluginCallback
Creates a Fastify plugin that monitors system metrics and responds with a 503 Service Unavailable status when the system is under pressure, as determined by the provided options and current metrics.
Parameters
- appName: Readonly<Partial<MiddlewareOptions>>
  The name of the application, formatted as ${string}-${string} (e.g., "service-order")
Returns FastifyPluginCallback
A Fastify plugin that checks system pressure and responds accordingly.
Remarks
When the system is detected to be under pressure, the plugin:
- Notifies observers with the current metrics.
- Sets the Retry-After header.
- Responds with HTTP 503 Service Unavailable. Otherwise, it continues processing the request.
Example
```
import Fastify from 'fastify';
import { underPressureFastifyPlugin } from '@stephen-shopopop/node-metrics';

const fastify = Fastify();

fastify.register(underPressureFastifyPlugin({
  sampleIntervalInMs: 1000,
  resolution: 10,
  webServerMetricsPort: 9090,
  maxEventLoopDelay: 1000,
  maxEventLoopUtilization: 0.9
}));
```
- Defined in library/middleware/fastify-under-pressure.ts:43