contrib/jackc/pgx.v5: publish pgxpool stats to statsd client

[bengentree]

Fixes #2680

What does this PR do?

Adds a new option to contrib/jackc/pgx.v5 to enable polling and submission of pgxpool stats as metrics to Datadog Ref: https://pkg.go.dev/github.com/jackc/pgx/v5/pgxpool#Stat

Motivation

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.
• [ ] 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.

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[github-actions[bot]]

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[darccio]

@rarguelloF @zarirhamza This looks good. Can you review it, please?