Add support for custom HTTP Transports for the otlphttp driver

[arnogeurts-sqills]

Problem Statement

Currently the HTTP transport for the otlphttp driver is hard coded and cannot be customised. It would be nice if a custom transport could be passed to the driver via an Option.

I'm looking into building/PoCing a sidecar proxy for my applications to forward the OTLP data to the central collector. For performance considerations I want to use a unix socket for communication between my application and sidecar proxy. A custom HTTP Transport will allow me to do this. Moreover, it will allow optimisation of other parameters like MaxIdleConns, IdleConnTimeout, KeepAlive, etc. when needed.

Proposed Solution

Add an Option "WithHTTPTransport" which allows specifying a custom HTTP transport for the otlphttp driver.

[MadVikingGod]

Could this be solved in a different manner?

One downside to this approach is that someone could use an already instrumented Transport creating a loop. That is currently not possible with our current configuration.