Rename "ot" to "otel"

[reyang]

OpenTelemetry specification has officially recommended "otel" as the abbreviation rather than "ot" (which could be confusing as it is used for OpenTracing).

Need to find all ot names and update them to otel when appropriate: https://github.com/open-telemetry/opentelemetry-dotnet/search?q="ot."&unscoped_q="ot."

Reference: https://github.com/open-telemetry/opentelemetry-specification#acronym

[eddynaka]

Assign me. i will create a PR replacing the strings

[eddynaka]

@reyang , for the case of the extensions, where we have ot.status_code, are we going to change that as well?

Or just the comments?
Thanks.

[reyang]

Looks like it will be all the cases returned by the query https://github.com/open-telemetry/opentelemetry-dotnet/search?q="ot."&unscoped_q="ot.