Improve expfmt.Format with expfmt.NegotiateAccept ergonomics

[bwplotka]

Problem

1. In PR #570, support for UTF-8 metric name escaping was introduced. This started to alter headers with extra parameters like escaping=allow-utf-8.
2. Because format strings now had dynamic parameters (like ; escaping=...), direct string equality checks (if format == expfmt.FmtText) started failing in downstream code and tests. To stop users from doing string comparisons, PR #576: Unexport Format strings
   unexported all Fmt... constants (making them fmtText, fmtProtoDelim, etc.) and introduced FormatType enum (TypeTextPlain, TypeProtoDelim) along with expfmt.NewFormat(FormatType).
3. Making things private immediately broke Kubernetes (k8s.io/component-base) and large portions of the Go Prometheus ecosystem. In response, PR #665: Revert #576 and add deprecation notice re-exported the constants, but marked every single one with Deprecated:. Since then community either updated to a NewFormat, or ignored the lint warnings.
4. In feat: implement gauge and counter support for OpenMetrics 2.0 #894 we introduced NegotiateAccept(h, FmtProtoDelim, FmtProtoText, FmtProtoCompact, FmtText) that allows full granularity of choices to accept. We couldn't use FormatType because it does not encapsulate version.

As a result NegotiateAccept API is now quite hard to use.

Examples of the current usage:

• config: upgrade sigv4 to v0.5.0 and document new fields prometheus#19569 (comment)

expfmt.NegotiateAccept(req.Header, 
  expfmt.NewFormat(expfmt.TypeProtoDelim), expfmt.NewFormat(expfmt.TypeProtoText), expfmt.NewFormat(expfmt.TypeProtoCompact), expfmt.NewFormat(expfmt.TypeTextPlain)
  )

• https://github.com/prometheus/client_golang/pull/2132/changes#r4073172974

var (
        standardAcceptedFormats    []expfmt.Format
	openMetricsAcceptedFormats []expfmt.Format
)

func init() {
	standardAcceptedFormats = []expfmt.Format{
		expfmt.NewFormat(expfmt.TypeProtoDelim),
		expfmt.NewFormat(expfmt.TypeProtoText),
		expfmt.NewFormat(expfmt.TypeProtoCompact),
		expfmt.NewFormat(expfmt.TypeTextPlain),
	}

	om20Format, err := expfmt.NewOpenMetricsFormat(expfmt.OpenMetricsVersion_2_0_0)
	if err != nil {
		panic(err)
	}
	om10Format, err := expfmt.NewOpenMetricsFormat(expfmt.OpenMetricsVersion_1_0_0)
	if err != nil {
		panic(err)
	}
	om001Format, err := expfmt.NewOpenMetricsFormat(expfmt.OpenMetricsVersion_0_0_1)
	if err != nil {
		panic(err)
	}
	openMetricsAcceptedFormats = []expfmt.Format{
		om20Format,
		om10Format,
		om001Format,
		expfmt.NewFormat(expfmt.TypeProtoDelim),
		expfmt.NewFormat(expfmt.TypeProtoText),
		expfmt.NewFormat(expfmt.TypeProtoCompact),
		expfmt.NewFormat(expfmt.TypeTextPlain),
	}
}

Goal

• NegotiateAccept is easier to use, especially on the hot path and for the known format sets