expfmt: precompute parsed goautoneg.Accept for well-known Format constants

[dashpole]

Fixes a small performance regression introduced by #894.

Repeatedly parsing static Format constants with goautoneg.ParseAccept in matchFormat on every call causes heap allocations during negotiation.

Precompute the parsed goautoneg.Accept structs for standard Format constants at package initialization time, falling back to goautoneg.ParseAccept for custom formats. Also add benchmarks with b.ReportAllocs() to track negotiation performance.

goos: linux
goarch: amd64
pkg: github.com/prometheus/common/expfmt
cpu: AMD EPYC 7B12
                                 │ /tmp/before.txt │           /tmp/after.txt            │
                                 │     sec/op      │   sec/op     vs base                │
Negotiate-24                         3.074µ ±   6%   2.240µ ± 2%  -27.10% (p=0.000 n=10)
NegotiateIncludingOpenMetrics-24     3.180µ ±   3%   2.314µ ± 2%  -27.22% (p=0.000 n=10)
NegotiateAccept-24                   3.138µ ± 153%   1.768µ ± 4%  -43.67% (p=0.000 n=10)
geomean                              3.130µ          2.093µ       -33.14%

                                 │ /tmp/before.txt │            /tmp/after.txt            │
                                 │      B/op       │     B/op      vs base                │
Negotiate-24                          1.524Ki ± 0%   1.086Ki ± 0%  -28.76% (p=0.000 n=10)
NegotiateIncludingOpenMetrics-24      1.759Ki ± 0%   1.320Ki ± 0%  -24.93% (p=0.000 n=10)
NegotiateAccept-24                     1417.0 ± 0%     968.0 ± 0%  -31.69% (p=0.000 n=10)
geomean                               1.548Ki        1.107Ki       -28.51%

                                 │ /tmp/before.txt │           /tmp/after.txt           │
                                 │    allocs/op    │ allocs/op   vs base                │
Negotiate-24                           15.000 ± 0%   9.000 ± 0%  -40.00% (p=0.000 n=10)
NegotiateIncludingOpenMetrics-24        16.00 ± 0%   10.00 ± 0%  -37.50% (p=0.000 n=10)
NegotiateAccept-24                     14.000 ± 0%   8.000 ± 0%  -42.86% (p=0.000 n=10)
geomean                                 14.98        8.963       -40.16%

[Copilot]

Pull request overview

This PR addresses a performance regression in expfmt content-type negotiation by avoiding repeated parsing of static Format constants with goautoneg.ParseAccept, reducing allocations and improving negotiation throughput.

Changes:

• Precompute parsed goautoneg.Accept values for well-known Format constants at package initialization, with a fallback parse for custom formats.
• Simplify escaping parameter construction in NegotiateAccept to avoid unnecessary formatting overhead.
• Add benchmarks (with allocation reporting) to track negotiation performance and catch regressions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
expfmt/encode.go	Adds a precomputed Format→Accept lookup to avoid per-call parsing in matchFormat, and simplifies escaping suffix construction.
expfmt/encode_test.go	Adds benchmarks for negotiation paths and reports allocations to guard against future regressions.

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[bwplotka]

Thanks