What does this PR do?

DBStats is disabled — and no metric submission is attempted from the database/sql contrib — when the contrib fails to create a statsd client. This can happen if, for example, the DogStatsD address granted to the tracer is an inaccessible unix domain socket.

Motivation

The contrib/database/sql was attempting metric submission if the DBStats feature was enabled, even if the statsd client initialization failed. This was causing a nil pointer exception when the nil statsd client was called upon to submit metrics (e.g, statsd.Gauge(...) where statsd is nil)

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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