Stabilize Prometheus Metrics Exporter spec: Content Negotiation

[johannaojeling]

What are you trying to achieve?

Stabilize this part of the Prometheus Metrics Exporter spec:

opentelemetry-specification/specification/metrics/sdk_exporters/prometheus.md

Lines 82 to 84 in 849edf7

	A Prometheus Exporter MUST support content negotiation to allow clients to request
	metrics in different formats based on the `Accept` header in HTTP requests. Content
	negotiation MUST follow [Prometheus Content Negotiation guidelines](https://prometheus.io/docs/instrumenting/content_negotiation/).

Additional context.

Related to: #4927

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[johannaojeling]

SDK implementations that support Accept-based content negotiation:

• .NET: PrometheusExporterMiddleware.cs, PrometheusHttpListener.cs - chooses OpenMetrics text or Prometheus text (text/plain with version=0.0.4) based on Accept.
• Go: exporter.go - uses prometheus/client_golang, which performs Accept-based format selection per Prometheus content negotiation.
• Java: PrometheusHttpServer.java - uses prometheus/client_java, which performs Accept-based format selection per Prometheus content negotiation.
• Python: prometheus/__init__.py - uses prometheus/client_python, which performs Accept-based format selection per Prometheus content negotiation.

SDK implementations that do not support Accept-based content negotiation:

• C++: exporter.cc - prometheus-cpp pull handler responds with fixed Content-Type: text/plain; charset=utf-8 and does not use Accept for format selection.
• JavaScript: PrometheusExporter.ts - HTTP handler responds with fixed Content-Type: text/plain and does not use Accept for format selection.