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

Signed-off-by: Lukas Juozas Janušaitis <lukas.janusaitis@hostinger.com>

Author: LukoJy3D

docs/instrumenting/exposition_formats.md

@@ -167,6 +167,22 @@ When such features are enabled either by feature flag
 appropriate configuration option (`scrape_native_histograms: true`) then
 Protobuf will be favored over other exposition formats.
 
+## HTTP Content-Type requirements
+
+Starting with Prometheus 3.0, scrape targets **must** return a valid `Content-Type` header for the metrics endpoint. If the `Content-Type` is missing, unparsable, or not a supported media type, **the scrape will fail**. See changes in [scrape protocols](https://prometheus.io/docs/prometheus/latest/migration/#scrape-protocols) in the migration guide for details.
+
+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.
+
 ## Historical versions
 
 For details on historical format versions, see the legacy