docs(exposition): clarify required Content-type header for scrapes

[LukoJy3D]

In prometheus/prometheus#16155, it was suggested to clarify it in exposition formats.

While writing it, I feel that some aspects overlap with those mentioned in https://prometheus.io/docs/instrumenting/content_negotiation/. Therefore, it may be sufficient to mention it in the migration guide, but I will give this a try.

[bwplotka]

Thanks!

This is great addition, but I propose to word things a bit differently, see the comment. WDYT? Is that helpful? (:

[bwplotka]

Hm, this is bit misleading. Content type depends on what you actually expose, but it reads here like it's a random string to put. (:

What if we make it clearer what the content type is for each section above?

Prometheus text format already mentions content type.

• In Proto section let's mention application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited
• In OpenMetrics section application/openmetrics-text;version=<version>

Version is NOT always optional. It matters. Especially with OpenMetrics, because there will be breaking changes between application/openmetrics-text;version=1.0.0 and (upcoming) application/openmetrics-text;version=2.0.0

Then we could say:

Suggested change

	At minimum, set the `Content-Type` media type to one of:
	- `application/openmetrics-text`
	- `text/plain`
	- `application/vnd.google.protobuf`
	Adding parameters such as `version=` (e.g. `text/plain;version=1.0.0`) is **optional**. Prometheus will correctly negotiate the protocol based solely on the media type. For the canonical list of protocols, parameters, and the negotiation rules, see [Scrape protocol content negotiation](./content_negotiation.md).
	### Common pitfall
	The value of the `Content-Type` header must be a **valid MIME type** — plain tokens like `OpenMetricsText1.0.0` are **not valid** and the scrape will fail. Use the full media type instead, `application/openmetrics-text;version=1.0.0'`. See [Protocol Headers](./content_negotiation.md#protocol-headers) for details.
	See each of the exposition format sections for the accurate HTTP content types.
	### ScrapeProtocols vs Content-Type
	Prometheus scrape config offers to configure negotiation based on the content-type using the [`scrape_protocols`](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config) config. For the Prometheus user convenience the scrape protocols are referenced by a unique name that maps to the concrete content-type. See the current (v3.7.3) mapping [here](https://github.com/prometheus/prometheus/blob/v3.7.3/config/config.go#L556).
	However, the targets should expose metrics in the exposition format with the absolute, response content-type (e.g. `application/openmetrics-text;version=1.0.0`) and only one.

[bwplotka]

Actually I just noticed https://prometheus.io/docs/instrumenting/content_negotiation lol, so maybe a bit simpler:

Suggested change

	At minimum, set the `Content-Type` media type to one of:
	- `application/openmetrics-text`
	- `text/plain`
	- `application/vnd.google.protobuf`
	Adding parameters such as `version=` (e.g. `text/plain;version=1.0.0`) is **optional**. Prometheus will correctly negotiate the protocol based solely on the media type. For the canonical list of protocols, parameters, and the negotiation rules, see [Scrape protocol content negotiation](./content_negotiation.md).
	### Common pitfall
	The value of the `Content-Type` header must be a **valid MIME type** — plain tokens like `OpenMetricsText1.0.0` are **not valid** and the scrape will fail. Use the full media type instead, `application/openmetrics-text;version=1.0.0'`. See [Protocol Headers](./content_negotiation.md#protocol-headers) for details.
	See each of the exposition format sections for the accurate HTTP content types.
	### ScrapeProtocols vs Content-Type
	Prometheus scrape config offers scrape protocol negotiation based on the content-type using the [`scrape_protocols`](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config) config. For the Prometheus user convenience the scrape protocols are referenced by a unique name that maps to the concrete content-type. See [Protocol Headers](./content_negotiation.md#protocol-headers) for details.
	However, the targets should expose metrics in the exposition format with the absolute, response content-type (e.g. `application/openmetrics-text;version=1.0.0`) and only one.