chore(contrib/valkey-io/valkey-go): Add db.instance to Valkey spans

[ysk24ok]

What does this PR do?

Adding db.instance field to Valkey spans.

Motivation

This is to align with inferred services. Without this, peer.hostname field (in a k8s environment the value is a pod name) and cardinality is too high.
db.name takes precedence over db.instance but in case of Valkey I was not sure what db.name represented so I chose db.instance.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[darccio]

Looks good for APM LP Go, but I relay on IDM approval.