Support HTTP protocol and TLS for OpenTelemetry traces exporter (#126)

* Currently, the OTEL exporter can't connect to HTTPS endpoints due to
missing `tls` and `tls-roots` features. This PR enables them.
* By default, OTEL exporter shouldn't be enabled if `--otlp-endpoint` is
empty. Currently the exporter is always running.
* Users can switch to HTTP protocol with
`OTEL_EXPORTER_OTLP_PROTOCOL=http/protobuf`
* Upgrade the latest dependencies for `opentelemetry` packages.
* Allow configuring log level with `RUST_LOG` env variable.