OTLP Exporter for Metrics (tonic) can cause deadlock

[cijothomas]

https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-otlp/src/exporter/tonic/metrics.rs#L2 uses std::sync::mutex. The exporter has await() points, and here it should use tokio::sync:mutex instead.

Discussed here: #2764 (comment)

[scottgerring]

Closing as dupe of #2802