Refactor refreshCache to remove anonymous func

[dmathieu]

Motivation/summary

As I am looking into adding instrumentation for this code path, I found this logic to be a bit obscure, especially as it shares variables between the main thread and an anonymous function.
So this refactor removes the anonymous function, so we explicitly pass the values we need.

Checklist

• NA Update CHANGELOG.asciidoc
• NA Update package changelog.yml (only if changes to apmpackage have been made)
• NA 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

This refactor has unit tests. There should be no behavior changes.

Related issues

I'm doing this while working on #1014, but not directly related to it either.

[mergify]

This pull request does not have a backport label. Could you fix it @dmathieu? 🙏
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]

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• Start Time: 2023-06-29T09:47:34.022+0000

• Duration: 17 min 25 sec

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[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                                      66.62n        66.56n       -0.09%

                                  │ 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                                                918.0n         889.7n        -3.08%

                                             │ 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                  448.2n         452.0n        +0.85%

               │ build/main/bench.out │              bench.out               │
               │         B/op         │     B/op      vs base                │
geomean                    380.4          379.1       -0.33%
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_big_tx/1000_events-12           678.6µ ±  11%    804.9µ ±  16%  +18.61% (p=0.026 n=6)
geomean                                              15.12µ           15.03µ          -0.60%

                                            │ build/main/bench.out │               bench.out               │
                                            │         B/op         │     B/op       vs base                │
ReadEvents/json_codec/10_events-12                   11.69Ki ±  0%   11.67Ki ±  0%  -0.23% (p=0.006 n=6)
geomean                                              11.31Ki         11.35Ki        +0.37%
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