[8.4] Fix race condition in custom libbeat instrumentation (backport #8900)

[mergify]

This is an automatic backport of pull request #8900 done by Mergify.
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	modified:   internal/beater/beater.go

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	deleted by us:   changelogs/head.asciidoc

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @lahsivjar? 🙏

[apmmachine]

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• Start Time: 2022-08-25T03:10:29.987+0000

• Duration: 25 min 37 sec

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Test	Results
Failed	0
Passed	137
Skipped	1
Total	138

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📚 Go benchmark report

Diff with the 8.4 branch

name                                                            old time/op    new time/op    delta
pkg:github.com/elastic/apm-server/internal/agentcfg goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/decoder goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/model/modelindexer goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/processor/stream goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/publish goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics goos:linux goarch:amd64
AggregateTransaction-12                                           71.3ns ± 0%    72.8ns ± 0%   +2.07%  (p=0.008 n=5+5)
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling goos:linux goarch:amd64
Process-12                                                        1.63µs ±41%    1.12µs ±24%  -31.48%  (p=0.016 n=5+5)
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage goos:linux goarch:amd64
WriteTransaction/nop_codec_big_tx-12                               855ns ± 6%     360ns ±49%  -57.93%  (p=0.016 n=4+5)

name                                                            old alloc/op   new alloc/op   delta
pkg:github.com/elastic/apm-server/internal/agentcfg goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/decoder goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/model/modelindexer goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/processor/stream goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/publish goos:linux goarch:amd64
Publisher-12                                                       294kB ± 1%     317kB ±12%   +7.86%  (p=0.016 n=4+5)
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling goos:linux goarch:amd64
Process-12                                                        10.1kB ± 7%     9.6kB ± 1%   -4.38%  (p=0.016 n=5+5)
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage goos:linux goarch:amd64
ReadEvents/json_codec/1000_events-12                              3.22MB ± 0%    3.19MB ± 0%   -1.05%  (p=0.029 n=4+4)

name                                                            old allocs/op  new allocs/op  delta
pkg:github.com/elastic/apm-server/internal/agentcfg goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/decoder goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/model/modelindexer goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/processor/stream goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/internal/publish goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage goos:linux goarch:amd64

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

[lahsivjar]

Target for 8.4.1 release

[lahsivjar]

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