Non-collectable resources in Exposer

[aeroyorch]

Hi!

We would like to serve additional non-Prometheus resources under the same HTTP server (e.g. typical health check (/healthz) and readiness (/readyz) endpoints) along with the metrics resources, without loosing the current functionality provided by the Exposerclass.

One possible solution would be using std::shared_ptr instead of std::unique_ptr to store the server_ member and having an additional constructor that accepts the provision of an externally managed CivetServer.

Thank you in advance.