OpenTelemetry tracing manual instrumentation support

In the same way as seen on the OpenTelemetry instrumentation for`go-redis` it would be extremely beneficial to create a similar package for redigo. **Not only we can track the overhead of the client code, but we can also precisely trace errors/latency spikes/timeouts/etc...**

Some further info:
- [Small example of goredis manual instrumentation](https://github.com/go-redis/redis/tree/master/extra/redisotel/example)
- [Full example of HTTP service + Redis backed (with client instrumentation)](https://github.com/filipecosta90/opentelemetry-go-http-sample)

## Important Note: Semantic conventions to follow while preparing a package for manual instrumentation.
While this is not mandatory, we should aim for standardization across clients/languages/DBs. Here's the spec on DB semantic conventions:

https://github.com/open-telemetry/semantic-conventions/blob/main/docs/database/redis.md

PS: will gladly contribute to this matter.