make span.setDbContext public

**Is your feature request related to a problem? Please describe.**

At the moment index.d.ts does not have span.setDbContext method while it is used internally in various db instrumentations.

**Describe the solution you'd like**

Add method to index.d.ts and to the documentation.

**Describe alternatives you've considered**

It is possible to add the required data in the span filter.

**Additional context**

I don't need the default instrumentation for db drivers and I want to start and fulfill spans manually. Therefore I need similar methods which are used by internal instrumentation. As I can see setDbContext allows to set the statement, instance and user. I don't need the statement field but want to see instance and user in my traces.