Add WithIdleConnTimeout HTTP client option

[austince]

This change enables HTTP long-polling SD mechanisms to configure the idle connection timeout so that they can use this config lib to create HTTP clients.

For example, the consul_sd manually creates a http.Transport:
https://github.com/prometheus/prometheus/blob/7bc11dcb06640ce22b4e15eb52b2c065f97cf79a/discovery/consul/consul.go#L191-L198

[austince]

@roidelapluie could you take a look at this when you get a chance?

[roidelapluie]

To be clear, do you plan to update consul_sd after this is merged? :)

I am asking because while we can accept some small contributions, the goal is not that we start maintaining a fully generic http client with all the options possible.

[austince]

To be clear, do you plan to update consul_sd after this is merged? :)

I'd be happy to :) and I'll use it in the updated kuma_sd as well prometheus/prometheus#8844

[austince]

@roidelapluie do you think this is an acceptable addition, since it has an immediate use case that will not continue to grow?

On a higher note, do you imagine all SDs in Prometheus that need an HTTP client should use this library?

[roidelapluie]

Yes, for kuma_sd ofc it's good

[austince]

Great, thank you!