fix(stats): allow WithStatsAttributes to override library defaults

[obitech]

Library defaults were appended after user attrs in recordStats, so any override for db.client.connection.pool.name supplied via WithStatsAttributes was silently dropped by attribute.NewSet's last-value-wins dedup. Seed both defaults (db.system.name and db.client.connection.pool.name) into statsOptions before options are applied so user attrs land at the end of the slice and win on key collision.

Add a regression test using OTel's manual reader that verifies user overrides take effect while non-overridden defaults are preserved.

Ref #71

[Copilot]

Pull request overview

This PR fixes stats metric attribute override behavior by ensuring library-supplied default attributes are applied before user-provided WithStatsAttributes, so user values win on key collisions (matching attribute.NewSet’s last-value-wins semantics). It also adds a regression test to confirm the override behavior while preserving non-overridden defaults.

Changes:

• Seed db.system.name and db.client.connection.pool.name into statsOptions.defaultAttributes before applying StatsOptions so user attributes can override defaults.
• Remove the late append of db.client.connection.pool.name during callback registration.
• Add a regression test using the OTel SDK manual reader to verify override semantics and default preservation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
meter.go	Moves default attribute construction earlier and ensures default pool-name attribute is included before user attrs so overrides work.
meter_test.go	Adds regression test verifying user-provided db.client.connection.pool.name overrides the library default while db.system.name remains the default.
go.mod	Adds explicit dependency on go.opentelemetry.io/otel/sdk/metric for the new manual-reader-based test.

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