### What does this PR do?

Implements the OTLP trace export pipeline (See: [RFC](https://docs.google.com/document/d/1AsUrJxjJavLvSG33kUAJLYzGU8IYgrnf1SMbFDsuGmo/edit?tab=t.0#heading=h.184fd4da9raa)) for dd-trace-go: when `OTEL_TRACES_EXPORTER=otlp` is set, the tracer converts Datadog spans into OTLP protobuf format and sends them directly to an OpenTelemetry Collector instead of the Datadog agent.

This adds three new components:

1. span_to_otlp.go — converts internal Datadog spans to OTLP [TracesData](https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/trace/v1/trace.proto#L28-L38) protobuf, including resource attributes, span kind/status, events, links, 128-bit trace IDs, and sampling filtering (only sampled spans are exported).
2. otlp_writer.go — a traceWriter implementation that buffers converted OTLP spans, marshals them to protobuf, and flushes with retry logic.
3. otlp_transport.go — a lightweight HTTP transport for sending protobuf payloads to an OTLP endpoint, separate from the existing Datadog-protocol transport.

The PR also renames the existing transport interface and config.transport field to ddTransport to make it clear that it handles Datadog-specific traffic (msgpack traces, stats), and ensures ddTransport always points at the Datadog agent even when OTLP mode is active.

### Motivation
We are adding native OTLP export support to dd-trace-go so users can send traces directly to any OTLP-compatible collector. A prior PR added the configuration layer that resolves `OTEL_TRACES_EXPORTER`, `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT`, and `OTEL_EXPORTER_OTLP_TRACES_HEADERS`. This PR builds the runtime pipeline on top of that configuration.

### IMPORTANT NOTE!
The existing `agentTraceWriter` and `ddTransport` was too tightly coupled to Datadog conventions — msgpack encoding, agent sampling rate feedback, stats collection — to extend it for the otlp path, so a separate writer and transport were introduced instead. The `otlpTraceWriter` owns its own `otlpTransport` directly (rather than reaching through `config.ddTransport`), keeping OTLP concerns isolated from the Datadog agent path. A follow-up PR may remove `ddTransport` from the config struct entirely, instead passing the transport directly to each writer and consumer at construction time. This would make all writer implementations structurally consistent — each owning its transport as a field — and eliminate the implicit coupling through config.

An additional PR to disable stats collection in otlp mode is in progress [here](#4609).

### Reviewer's Checklist
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- [ ] Changed code has unit tests for its functionality at or near 100% coverage.
- [ ] [System-Tests](https://github.com/DataDog/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.

Unsure? Have a question? Request a review!


Co-authored-by: kakkoyun <[email protected]>
Co-authored-by: moezein0 <[email protected]>
Co-authored-by: darccio <[email protected]>
Co-authored-by: kemal.akkoyun <[email protected]>

### What does this PR do?

Introduces a `statsConcentrator` interface with a `noopConcentrator` implementation so the tracer skips stats computation in OTLP export mode.

- `tracer.stats` is now a `statsConcentrator` interface instead of a concrete `*concentrator.` In OTLP mode it's assigned a `noopConcentrator`; otherwise a real concentrator.
- The non-blocking channel send to `concentrator.In` was moved into a new `trySendSpan` method since interface fields can't expose struct fields — this also encapsulates the channel as an internal detail.

### Motivation:

In OTLP export mode, Datadog-format stats shouldn't be computed or sent. This approach uses a no-op behind an interface so the disabled case is type-safe and can't be broken by forgetting a guard.

### Reviewer's Checklist
<!--
* Authors can use this list as a reference to ensure that there are no problems
  during the review but the signing off is to be done by the reviewer(s).
-->

- [ ] Changed code has unit tests for its functionality at or near 100% coverage.
- [ ] [System-Tests](https://github.com/DataDog/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.

Unsure? Have a question? Request a review!


Co-authored-by: kakkoyun <[email protected]>
Co-authored-by: moezein0 <[email protected]>
Co-authored-by: darccio <[email protected]>