Warn on intitialization of Simple SpanProcessor

[Aneurysm9]

Problem Statement

The simpleSpanProcessor is not intended for use outside of development and debugging. However, many early examples and demos included its use and users report with some regularity performance issues caused by its use.

Proposed Solution

We should emit a warning when NewSimpleSpanProcessor() is invoked.

Alternatives

Documentation and examples have already been updated to use a batch span processor.

[MrAlias]

We should likely standardize on a logging solution before this so we do not end up with disparate solutions here: #1068

[dubonzi]

Hi, I'd like to work on this.

Looking at the global logger implementation, there isn't a Warn function, would Info be sufficient in this case?

[pellared]

@dubonzi I think the API can be extended to have a Warn function

[dubonzi]

@pellared Would that have to be on it's own issue?

[pellared]

It can be part of this issue.