chore(config): migrate sendRetries

[mtoffl01]

What does this PR do?

Migrates tracer to using Config.sendRetries rather than its own config.sendRetries

Motivation

datadoghq.atlassian.net/browse/APMAPI-1748

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!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-12-10 16:37:04

Comparing candidate commit 30bec03 in PR branch mtoff/sendRetries with baseline commit b760677 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 15 metrics, 0 unstable metrics.

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

<sub>This comment will be updated automatically if new data arrives.
* Fix with Cursor requires Datadog plugin ≥v2.17.0
🔗 Commit SHA: 30bec03 | Docs | Datadog PR Page | Was this helpful? Give us feedback!</sub>

[mtoffl01]

For changes to this test, see: https://dd.slack.com/archives/C03D7LNP0TD/p1765313037158839

by newly importing internalconfig "github.com/DataDog/dd-trace-go/v2/internal/config", I believe that triggered a dependency tree that caused processtags init() function to run earlier than it did before. As a result, processtags.enabled evaluated to true (by default), and process tags were appended onto span metadata, causing the payload size to increase from 185 bytes to 308.