feat: migrate metadata decoding to protobuf

[kruskall]

Motivation/summary

Last "big" change related to the protobuf work:

• migrate metadata decoding to protobuf
• update leftover tests to use modelpb

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Update package changelog.yml (only if changes to apmpackage have been made)
- [ ] Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

Blocked by elastic/apm-data#67
Blocked by #11013
Related to elastic/apm-data#52

[mergify]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[apmmachine]

💚 Build Succeeded

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• Start Time: 2023-06-19T09:16:24.614+0000

• Duration: 6 min 7 sec

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[mergify]

This pull request is now in conflicts. Could you fix it @kruskall? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b feat/migrate-metadata-pb upstream/feat/migrate-metadata-pb
git merge upstream/main
git push upstream feat/migrate-metadata-pb

[apmmachine]

📚 Go benchmark report

Diff with the main branch

goos: linux
goarch: amd64
pkg: github.com/elastic/apm-server/internal/agentcfg
cpu: 12th Gen Intel(R) Core(TM) i5-12500
                                  │ build/main/bench.out │             bench.out             │
                                  │        sec/op        │   sec/op     vs base              │
geomean                                      67.80n        67.66n       -0.21%

                                  │ build/main/bench.out │             bench.out              │
                                  │         B/op         │    B/op     vs base                │
geomean                                                ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                                  │ build/main/bench.out │             bench.out              │
                                  │      allocs/op       │ allocs/op   vs base                │
geomean                                                ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

pkg: github.com/elastic/apm-server/internal/beater/request
                                             │ build/main/bench.out │             bench.out              │
                                             │        sec/op        │    sec/op     vs base              │
geomean                                                879.2n         851.8n        -3.11%

                                             │ build/main/bench.out │              bench.out               │
                                             │         B/op         │     B/op      vs base                │
geomean                                                           ²                 +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                                             │ build/main/bench.out │             bench.out              │
                                             │      allocs/op       │ allocs/op   vs base                │
geomean                                                           ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

pkg: github.com/elastic/apm-server/internal/publish
             │ build/main/bench.out │          bench.out          │
             │        sec/op        │   sec/op    vs base         │

             │ build/main/bench.out │           bench.out            │
             │         B/op         │     B/op       vs base         │

             │ build/main/bench.out │           bench.out           │
             │      allocs/op       │  allocs/op    vs base         │

pkg: github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics
                 │ build/main/bench.out │          bench.out           │
                 │        sec/op        │   sec/op     vs base         │

                 │ build/main/bench.out │            bench.out            │
                 │         B/op         │     B/op      vs base           │
¹ all samples are equal

                 │ build/main/bench.out │           bench.out           │
                 │      allocs/op       │ allocs/op   vs base           │
¹ all samples are equal

pkg: github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics
                        │ build/main/bench.out │          bench.out           │
                        │        sec/op        │   sec/op     vs base         │

                        │ build/main/bench.out │           bench.out           │
                        │         B/op         │    B/op     vs base           │
¹ all samples are equal

                        │ build/main/bench.out │           bench.out           │
                        │      allocs/op       │ allocs/op   vs base           │
¹ all samples are equal

pkg: github.com/elastic/apm-server/x-pack/apm-server/sampling
               │ build/main/bench.out │             bench.out              │
               │        sec/op        │    sec/op     vs base              │
geomean                  461.6n         425.8n        -7.75%

               │ build/main/bench.out │              bench.out               │
               │         B/op         │     B/op      vs base                │
geomean                    379.5          378.0       -0.38%
¹ all samples are equal

               │ build/main/bench.out │             bench.out              │
               │      allocs/op       │ allocs/op   vs base                │
geomean                    4.123        4.123       +0.00%
¹ all samples are equal

pkg: github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage
                                            │ build/main/bench.out │              bench.out              │
                                            │        sec/op        │    sec/op      vs base              │
ReadEvents/nop_codec/1_events-12                     1.724µ ±  15%   1.575µ ±   7%  -8.64% (p=0.026 n=6)
geomean                                              14.87µ          14.53µ         -2.29%

                                            │ build/main/bench.out │               bench.out                │
                                            │         B/op         │     B/op       vs base                 │
ShardedWriteTransactionContended-12                    882.5 ± 11%    1053.0 ± 35%  +19.32% (p=0.041 n=6)
geomean                                              11.30Ki         11.35Ki         +0.43%
¹ all samples are equal

                                            │ build/main/bench.out │              bench.out              │
                                            │      allocs/op       │  allocs/op   vs base                │
geomean                                                 127.4         127.4       -0.00%
¹ all samples are equal

report generated with https://pkg.go.dev/golang.org/x/perf/cmd/benchstat