expfmt: fix OpenMetrics 2.0 decoder error, format docs, and encoder version dispatch

[dashpole]

Some small follow-ups to the OpenMetrics 2.0 support introduced by #894.

• Return unsupported format error in NewDecoder for OpenMetrics 2.0.
• Clarify experimental encode-only status and ignored EncoderOptions in doc comments.
• Use mime.ParseMediaType in NewEncoder to properly handle cases where a format or Content-Type header uses standard MIME quoting or spacing (e.g. application/openmetrics-text; version="2.0.0").

[Copilot]

Pull request overview

This PR applies follow-up fixes and clarifications to the expfmt OpenMetrics 2.0 support introduced in #894, focusing on correct format/version dispatch, clearer “encode-only experimental” messaging, and more explicit decode behavior for OpenMetrics 2.0.

Changes:

• Make NewEncoder (and NewDecoder for OpenMetrics) use mime.ParseMediaType so version parameters are correctly handled even with standard MIME quoting/spacing.
• Return an explicit “not supported for decoding” error from NewDecoder for OpenMetrics version=2.0.0.
• Clarify documentation around OpenMetrics 2.0’s experimental/encode-only status and option handling; add tests covering the new dispatch/error behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
expfmt/openmetrics_2_0_create.go	Clarifies OpenMetrics 2.0 encoder documentation and explicitly ignores encoder options.
expfmt/expfmt.go	Documents experimental/encode-only status in OpenMetrics format construction and comments.
expfmt/encode.go	Uses mime.ParseMediaType to dispatch OpenMetrics encoding by version parameter reliably.
expfmt/encode_test.go	Adds coverage ensuring OpenMetrics 2.0 encoder dispatch works with reordered/quoted MIME params.
expfmt/decode.go	Returns an explicit unsupported decode error for OpenMetrics 2.0 based on version parameter.
expfmt/decode_test.go	Adds coverage verifying NewDecoder errors for OpenMetrics 2.0 and succeeds/falls back elsewhere.

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