[Geneva exporter] Handle double, int64 and bool types properly while encoding

[lalitb]

I just followed C++ implementation for now - it uses BT_FLOAT for double/float. But agreed, this is not right. OTLP supports only double, and better to encode it as double. Have added a TODO for this, and also created an issue to track this.

Originally posted by @lalitb in #261 (comment)