Benchmark the custom ITextFormatter path

[mishamyte]

First half of #349.

SinkBenchmarks never passes textFormatter — both its real-target and fake-transport branches use the default — so every benchmarked configuration takes the built-in LokiJsonTextFormatter fast path. FormatterBenchmarks measures the public Format surface, which the sink does not use in production (its own comment says so).

So the custom-ITextFormatter route through the serializer (Utf8TextWriter over the pooled body buffer, rather than FormatToBuffer) has no benchmark coverage at all, and an allocation regression on it is invisible to the Allocated signal CI relies on. That blind spot is how #347 shipped.

Change

Adds a CustomFormatterSinkBenchmarks group driving the same end-to-end pipeline through MessageTemplateTextFormatter with Serilog's stock output template — the most common custom formatter, and the shape reported in #347.

Added as a separate group rather than a Formatter param on SinkBenchmarks deliberately: a new param would rename all four existing rows (they would gain Formatter: "Default"), breaking pairing with published baselines and with the numbers in earlier PR comments. A separate class leaves them byte-identical and adds two rows instead of four.

Setup common to both groups is extracted into a private SinkSetup module. Applied to both the Current and NuGet projects — Benchmarks.fs is per-project (only EventGen/BenchConfig/Program are shared), and the method names and params must match for compare-results.fsx, which joins rows by FullName.

Measured locally

| Method | EventCount | Payload   | Mean      | Allocated |
|------- |----------- |---------- |----------:|----------:|
| Push   | 1000       | Exception | 23.109 ms | 7573.8 KB |
| Push   | 1000       | Simple    |  2.810 ms |  116.03 KB |

Baseline and source both allocate a Utf8TextWriter per event at this point, so this PR should show ~0 delta between them. It establishes the number that the second half of #349 then improves.

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📥 Commits

Reviewing files that changed from the base of the PR and between 5a8418f and 9b0afd1.

📒 Files selected for processing (2)

• benchmarks/Serilog.Sinks.Grafana.Loki.Benchmarks.Current/Benchmarks.fs
• benchmarks/Serilog.Sinks.Grafana.Loki.Benchmarks.NuGet/Benchmarks.fs

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Benchmark comparison

Baseline: Serilog.Sinks.Grafana.Loki 9.0.1 (latest on NuGet) vs this source.

Allocated is exact and deterministic — treat it as the regression signal.
Mean on shared CI runners is noisy; deltas within ±10% are not meaningful.

Benchmark	v9.0.1 alloc	source alloc	Δ alloc	v9.0.1 mean	source mean	Δ mean
CustomFormatterSinkBenchmarks.Push(EventCount: 1000, Payload: "Exception")	8.44 MB	8.48 MB	+0.5%	12.53 ms	11.31 ms	−9.7%
CustomFormatterSinkBenchmarks.Push(EventCount: 1000, Payload: "Simple")	120.0 KB	108.0 KB	−10.0% 🟢	2.10 ms	3.39 ms	+61.8% 🔺
SinkBenchmarks.Push(EventCount: 1000, Payload: "Exception")	7.71 MB	7.79 MB	+1.0%	37.23 ms	18.40 ms	−50.6% 🟢
SinkBenchmarks.Push(EventCount: 1000, Payload: "Simple")	155.3 KB	136.2 KB	−12.3% 🟢	5.25 ms	6.93 ms	+31.9% 🔺
SinkBenchmarks.Push(EventCount: 10000, Payload: "Exception")	77.19 MB	77.80 MB	+0.8%	114.67 ms	106.49 ms	−7.1%
SinkBenchmarks.Push(EventCount: 10000, Payload: "Simple")	1.36 MB	1.36 MB	+0.0%	18.33 ms	15.96 ms	−12.9% 🟢