[monitoring] Add OpenTelemetry spans in scd/datastore/subscriptions

[the-glu]

This is a set of 3 PRs, proposition to add OpenTelemetry to generate optional traces.
You can check the base PR, the one for handlers and the one for datastores.

This PR add as an example spans in pkg/scd/store/datastore/subscriptions.go

Spans look like that:

Idea would be to have that in all datastores operations (that have the context).

[mickmis]

Reviewed 7f9fcec
Same as for #1366: it is quite noisy to have all those additional calls in every single function. It is also error-prone as in the future it is too easy to miss adding some.

IMO we should not need to trace internal functions. Here since we have an interface Subscription we could have a tracing interface wrapping this one.
Also rather than hardcoding the name of the function manually we could be using reflect to get the function's name.

[the-glu]

Same as #1366 let close that one for now, and if possible add if we have a better store ^^'