OTLP Exporter - reuse buffers

[cijothomas]

OTLP Exporters are quite heavy in terms of allocations today. In-Memory Log/Span/Metric is converted to tonic/proto Structs, which requires ownership and forces clone. This is serialized into vec The compression (in http), is done to a new vec The ownership of the vec is passed to exporting client, forcing re-creation of it for each export.

Opening an issue to investigate and improve this.

Related: https://github.com/open-telemetry/opentelemetry-rust/pull/2485