Database semconv stability migration guide

[trask]

Fixes #1022

Changes

Documents the changes between experimental database semantic conventions and the (to-be) stable database semantic conventions.

[trask]

TODO: add section to document changes to experimental metrics also

[maryliag]

TODO: add section to document changes to experimental metrics also

example: db.client.connections.usage -> db.client.connection.count

[trask]

I'm thinking we should wait to merge this until after stability, just to avoid confusion in case there are any last-minute changes.

But please review it and leave comments / suggestions / questions.

[TylerHelmuth]

Saw that Database semconv was getting ready to mark stable and I love that this has existed along side it ❤️

[trask]

@open-telemetry/specs-semconv-maintainers the policy violations seem unrelated to this PR:

Diagnostic report:

Violation: Resource 'service' cannot remove required/recommended attributes (missing '{"service.name", "service.version"}')
  - Category         : backward_compatibility
  - Type             : semconv_attribute
  - SemConv group    : service
  - SemConv attribute: 
  - Provenance: /home/weaver/source

Violation: Resource 'telemetry.sdk' cannot remove required/recommended attributes (missing '{"telemetry.sdk.language", "telemetry.sdk.name", "telemetry.sdk.version"}')
  - Category         : backward_compatibility
  - Type             : semconv_attribute
  - SemConv group    : telemetry.sdk
  - SemConv attribute: 
  - Provenance: /home/weaver/source