transport: Introduce NewHTTPTransportOptions

[marclop]

Description

Introduces a new transport.NewHTTPTransportOptions() function which
accepts an Options struct to be able to customize the httptransport
without using environment variables

[apmmachine]

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• Start Time: 2022-01-10T10:52:25.671+0000

• Duration: 33 min 41 sec

• Commit: 01e0471

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[axw]

IIANM this will panic if HTTPTransportOptions.ServerURLs is empty. This should probably also return an error.

Maybe worth adding a Validate method, like:

func (o HTTPTransportOptions) Validate() error {
    if len(o.ServerURLs) == 0 {
        // ...
    }
    if len(o.ServerTimeout) < 0 {
        // ...
    }
    return nil
}

[simitt]

In my opinion NewHTTPTransport and NewHTTPTransportOptions should deliever the same default values for non-customized options, so also ensure that the same default values for ServerURLs and ServerTimeout are set.
Can you please add a test for when using NewHTTPTransportOptions?

[axw]

LGTM, just a couple of minor things