Add pretty print config option to OTLP file exporter

[jack-berg]

This has been requested / implemented in opentelemetry-java.

I propose adding a pretty print boolean config option to the file exporter, which defaults to false, and which if set to true, results in a the types of human-readable JSON formatting we're accustomed to in our editors / online formatting tools / JSON.stringify(data, null, 2). I would propose no standard definition for what pretty print entails - there is no standard and so I would want to optimize for ease of implementation across languages.

Setting this option to true would ruin the single line JSON feature which makes the output convenient for machines to parse, and so is most useful in local debugging situations.