fixes(configuration): Not unique exporter for exporter type

[workgroupengineering]

fixes #1700

This PR

• Don't contain the break change
• It has an integration test

[AndreyAkinshin]

I don't think that the logging approach is the best way to report a warning in this case. In most cases, BenchmarkDotNet users read only the bottom part of the log (with the summary table) and don't scroll up. Thus, this warning will be most likely lost.
On the other hand, we could define IAnalyser which reports such a warning. The IAnalyser approach has two advantages:

1. It prints all the warning below the summary table (so the warnings are more visible for the users)
2. The list of the warnings could be fetched programmatically (so it's possible to introduce an assertion that the benchmarks don't produce any warnings).

However, this particular case is tricky because we have to report warning on the stage of config building (not at the final stage of the BenchmarkReport analysis). I guess, we could introduce an IReadOnlyList<Conclusion> property in IConfig and use it to report issues related to malformed config in a structured way.

@adamsitnik what do you think?

[workgroupengineering]

Please @AndreyAkinshin can you review? If the proposed changes are not to your liking. Please close this PR. 😄

[AndreyAkinshin]

Closed since #1796 looks better.