Missing context in WithInstance method

[maksim77]

Is your feature request related to a problem? Please describe.
If the db.DB object is instrumented to work with Opentelemtry, the postgres.WithInstance method spawns a span (Ping method) learned from context.Background which is thus not bound to any trace.

Describe the solution you'd like
It would probably be cool to have a method like WithInstanceContext or something similar that takes a context as an argument and uses it in particular in the Ping method call

[joschi]

Kind of related to #1132.