How to deal with cargo features of HTTP client crates?

[frigus02]

opentelemetry-application-insights requires users to bring an HTTP client. opentelemetry-http brings implementations for reqwest, surf and isahc currently. But users are free to implement the corresponding trait for other HTTP clients, too. Each HTTP client crate may have cargo features.

opentelemetry-application-insights started adding "proxy" features for some reqwest features: https://github.com/frigus02/opentelemetry-application-insights/blob/1e5727a79c6d59631d640854879e05116cd1bc87/Cargo.toml#L29-L30

But doing that for all clients and all their features doesn't seem maintainable.

I would like to:

• Figure out how users are supposed to enable features for the HTTP client crate of their choosing.
• Make this clear through documentation.