fix(ddtrace/tracer): honor DD_DOGSTATSD_PORT over agent StatsD port

[SergioChan]

What does this PR do?

• Updates resolveDogstatsdAddr so an explicitly configured DD_DOGSTATSD_PORT is preserved instead of being overwritten by the agent-reported StatsD port.
• Keeps existing behavior for UDS and for cases where DD_DOGSTATSD_PORT is not set.
• Updates TestTracerOptionsDefaults/dogstatsd expectations for env-port and env-all to match the new precedence.
• Updates the WithDogstatsdAddr docs to reflect the new precedence rule.

Motivation

Issue #4518 reports that DD_DOGSTATSD_PORT is ignored when the agent reports a port, which prevents users from enforcing a different exposed DogStatsD port.

This change gives precedence to explicit DD_DOGSTATSD_PORT configuration while keeping the agent-reported port behavior as the default fallback.

Fixes #4518

Validation:

• Attempted: go test ./ddtrace/tracer -run "dogstatsd/env-(port|all)$" -count=1
• Result: not runnable in current upstream state due unrelated compile errors in existing tests (option_test.go / writer_test.go referencing removed config fields like agentURL / traceProtocol).

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 make lint locally.
• New code doesn't break existing tests. You can check this by running make test locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• All generated files are up to date. You can check this by running make generate locally.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running make fix-modules locally.

[rarguelloF]

@SergioChan thanks so much for the PR!

Sorry it took a little bit longer than expected, but fixing this issue was already in our radar. I went ahead and did a deeper refactor so all the user provided configuration (DD_DOGSTATSD_HOST, DD_DOGSTATSD_PORT and WithDogstatsdAddr) is used before the discovered/default configurations: #4575

[SergioChan]

Thanks for the update and for linking #4575. Since that broader refactor covers this issue path, I’m closing this PR to avoid duplicate work.

[SergioChan]

Closing as superseded by #4575, which applies the user-provided DogStatsD config precedence more comprehensively.