remove labelstore from prometheus interceptor

[kgeckhart]

PR Description

Removes unnecessary usage of labelstore from the prometheus.interceptor used in prometheus pipelines.

Beyond simplifying the code there's a performance gain from the reduction in duplicate staleness tracking through the labelstore,

goos: darwin
goarch: arm64
pkg: github.com/grafana/alloy/internal/component/prometheus
cpu: Apple M3 Pro
                                  │   baseline   │                new                 │
                                  │    sec/op    │   sec/op     vs base               │
Pipelines/default/2-metrics-11      5.638µ ±  7%   5.297µ ± 3%   -6.05% (p=0.009 n=6)
Pipelines/relabel/2-metrics-11      6.267µ ±  1%   5.819µ ± 2%   -7.14% (p=0.002 n=6)
Pipelines/default/10-metrics-11     27.78µ ±  6%   25.79µ ± 1%   -7.17% (p=0.002 n=6)
Pipelines/relabel/10-metrics-11     32.76µ ±  7%   29.71µ ± 2%   -9.29% (p=0.002 n=6)
Pipelines/default/100-metrics-11    309.6µ ± 10%   271.5µ ± 3%  -12.31% (p=0.002 n=6)
Pipelines/relabel/100-metrics-11    350.0µ ±  2%   308.3µ ± 5%  -11.92% (p=0.002 n=6)
Pipelines/default/1000-metrics-11   3.099m ±  8%   2.772m ± 3%  -10.57% (p=0.002 n=6)
Pipelines/relabel/1000-metrics-11   3.644m ±  4%   3.228m ± 3%  -11.44% (p=0.002 n=6)
geomean                             118.7µ         107.4µ        -9.52%

                                  │   baseline   │                 new                 │
                                  │     B/op     │     B/op      vs base               │
Pipelines/default/2-metrics-11      1.926Ki ± 0%   1.644Ki ± 0%  -14.66% (p=0.002 n=6)
Pipelines/relabel/2-metrics-11      2.505Ki ± 0%   2.082Ki ± 0%  -16.90% (p=0.002 n=6)
Pipelines/default/10-metrics-11     9.629Ki ± 0%   8.217Ki ± 0%  -14.66% (p=0.002 n=6)
Pipelines/relabel/10-metrics-11     12.52Ki ± 0%   10.41Ki ± 0%  -16.88% (p=0.002 n=6)
Pipelines/default/100-metrics-11    98.36Ki ± 0%   84.27Ki ± 0%  -14.33% (p=0.002 n=6)
Pipelines/relabel/100-metrics-11    126.6Ki ± 0%   105.5Ki ± 0%  -16.68% (p=0.002 n=6)
Pipelines/default/1000-metrics-11   997.1Ki ± 0%   856.4Ki ± 0%  -14.11% (p=0.002 n=6)
Pipelines/relabel/1000-metrics-11   1.249Mi ± 0%   1.043Mi ± 0%  -16.50% (p=0.002 n=6)
geomean                             41.76Ki        35.24Ki       -15.60%

                                  │  baseline   │                new                 │
                                  │  allocs/op  │  allocs/op   vs base               │
Pipelines/default/2-metrics-11       46.00 ± 0%    42.00 ± 0%   -8.70% (p=0.002 n=6)
Pipelines/relabel/2-metrics-11       58.00 ± 0%    52.00 ± 0%  -10.34% (p=0.002 n=6)
Pipelines/default/10-metrics-11      230.0 ± 0%    210.0 ± 0%   -8.70% (p=0.002 n=6)
Pipelines/relabel/10-metrics-11      290.0 ± 0%    260.0 ± 0%  -10.34% (p=0.002 n=6)
Pipelines/default/100-metrics-11    2.300k ± 0%   2.100k ± 0%   -8.70% (p=0.002 n=6)
Pipelines/relabel/100-metrics-11    2.900k ± 0%   2.600k ± 0%  -10.34% (p=0.002 n=6)
Pipelines/default/1000-metrics-11   24.49k ± 0%   22.49k ± 0%   -8.17% (p=0.002 n=6)
Pipelines/relabel/1000-metrics-11   30.49k ± 0%   27.49k ± 0%   -9.84% (p=0.002 n=6)
geomean                              985.0         892.5        -9.40%

The results were generated through the "headless" prometheus pipeline tests I added to track end-to-end scenarios related to WAL functionality that will be important as the labelstore continues to evolve.

Notes to the Reviewer

Fanout is the appender that must be used to emit metrics from any alloy component as it's what ensures we can multi-cast metrics to downstream consumers. The interceptor is used by most components that emit metrics but it is not a requirement, enrich + relabel use it to run their functionality, and scrape uses it to support livedebugging but it's not used by receive_http, otelcol.exporter.prometheus, and the operator which all use Fanout directly. Since any component that needs to emit metrics uses Fanout it makes the most sense to put logic there to enforce emitted metrics have a valid global seriesRef / are properly tracked for staleness.

PR Checklist

• CHANGELOG.md updated
• Tests updated

[kgeckhart]

This is covered through the relabel pipeline test

[ptodev]

I'm not super familiar with the label store, but on a high level this change makes sense. I'm just not sure why the Fanout is a better place for a cache than Interceptor? In theory should the Interceptor be better, since it's the entry point to a component?

[ptodev]

Does this change really need a changelog entry? It is more like an internal refinment that doesn't impact users.

[kgeckhart]

🤷 it's an optimization on a highly used pipeline

[ptodev]

The issue is that the entry does say how the change impacts end users. If it's an optimisation I'd expect it to say something like "improved the CPU performance of prometheus.* components".

[kgeckhart]

I'm just not sure why the Fanout is a better place for a cache than Interceptor? In theory should the Interceptor be better, since it's the entry point to a component?

It's a good question that I didn't expand upon in the PR. Fanout is the appender that must be used to emit metrics from any alloy component as it's what ensures we can multi-cast metrics to downstream consumers. The interceptor is used by most components that emit metrics but it is not a requirement, enrich + relabel use it to run their functionality, and scrape uses it to support livedebugging but it's not used by receive_http, otelcol.exporter.prometheus, and the operator which all use Fanout directly. Since any component that needs to emit metrics uses Fanout it makes the most sense to put logic there to enforce emitted metrics have a valid global seriesRef.

I added a comment to Fanout to indicate it's role with global series refs and updated the PR description with the further context about why Fanout.

[ptodev]

Thank you, LGTM but I think the changelog entry could be more clear.

[ptodev]

The issue is that the entry does say how the change impacts end users. If it's an optimisation I'd expect it to say something like "improved the CPU performance of prometheus.* components".