Add prometheus namespace validation

[Choraden]

go run ./cmd/forwarder run --prom-namespace upstream-proxy
Error: invalid argument "upstream-proxy" for "--prom-namespace" flag: invalid namespace: upstream-proxy, it must match "^[a-zA-Z_:][a-zA-Z0-9_:]*$"
See 'forwarder run --help' for usage.
exit status 1

[what-the-diff]

PR Summary

• Customization for Prometheus Namespace
  The ability to specify a custom Prometheus namespace for metrics has been introduced. This is a result of an update in the flag.go file that now accepts such specified values.

• Addition of Namespace Validation
  A new function, ParsePrometheusNamespace, has been added to check the validity of the entered Prometheus namespace. This function, located in the config.go file, ensures the namespace adheres to the required format.

• Inclusion of Namespace Validation in HTTP Server Configuration
  An additional layer of validation is added to the HTTP Server configuration, to further confirm that the Prometheus namespace is valid. If the namespace is found to be invalid during this check, an error will be returned promptly.

[mmatczuk]

It feels like http_server: validate prometheus namespace goes beyond scope of http server validation.

I'd prefer to limit that to binding.

[mmatczuk]

As a global var we'd benefit from comptime validation.

[Choraden]

It's global now.

[Choraden]

I limited the verification to binding only.