log/sdk: Add benchmarks

[pellared]

[MrAlias]

From SIG meeting: this looks almost complete. @dmathieu plans to another audit and review all coverage.

[dmathieu]

From the public SDK methods, we are now benchmarking:

• Record.WalkAttributes()
• Record.SetAttributes
• Record.AddAttributes
• LoggerProvider.Logger
• logger.newRecord
• SimpleProcessor.OnEmit
• BatchProcessor.OnEmit
• Create a record, with body, severity, attributes, and run SimpleProcessor.OnEmit
• Create a record, with body, severity, attributes, and run BatchProcessor.OnEmit
• Create a record, with body, severity, attributes, modify the observed timestamp and run SimpleProcessor.OnEmit
• Create a record, with body, severity, attributes, modify the observed timestamp and run BatchProcessor.OnEmit

At this stage, all hot public APIs appear to be benchmarked in the log SDK. I believe this issue can be closed.

[pellared]

We can always add more benchmarks later. Great job.