middleware

Here are a couple of example middlewares for natsmicromw. The middlewares are:

• requestid.go: Request ID middleware that parses the header for a request id (storing it into the request context), or generates a unique ID for each request using the header tag 'request_id' or a specified header tag.
• metrics.go: Metrics middleware that collects Prometheus metrics for NATS messages including the total number of messages received, duration of requests, and size of payloads.