[exporter/nats] add NATS exporter

[leocarrozzo]

Description

This PR implements the NATS Exporter component for OpenTelemetry Collector (exporter/natsexporter).
It provides support for Traces, Metrics, and Logs over NATS Core and JetStream, with both OTLP Protobuf and JSON encodings.

To keep the review process manageable and focused, this PR introduces the Exporter first. The NATS Receiver will be submitted in a separate follow-up PR.

I am also volunteering as Code Owner for exporter/natsexporter.

Link to tracking issue

Partially resolves #39540

Testing

• Unit and integration tests added under exporter/natsexporter.
• Linter checks passed locally (make lint).
• Manual end-to-end verification performed using telemetrygen and a local NATS server instance.

Documentation

• Added README.md and metadata.yaml for exporter/natsexporter.

Authorship

• I, a human, wrote this pull request description myself.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: leocarrozzo / name: Leonardo Carrozzo (82b42be, 9acad5e, f916c0e)

[github-actions]

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Pull request dashboard status

Status last refreshed: 2026-07-23 00:30:20 UTC.

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[leocarrozzo]

FYI on Architecture and Design:
The design and implementation of the NATS exporter takes reference from the existing Kafka and RabbitMQ exporters. This ensures full consistency with established OpenTelemetry standards, idiomatic code patterns, and maintainability.