fix(otel): cents-scale bucket boundaries for session.cost.total histogram

[TestardR]

Problem

opencode.session.cost.total is a Histogram registered without explicit bucket boundaries, so the OTel JS SDK applies defaults [0, 5, 10, 25, 50, 75, 100, 250, 500, 750, 1000, 2500, 5000, 7500, 10000] (USD-scale).

Real-world session costs are typically under $1, so every observation lands in the le=5 bucket. P50/P95 panels return 0 because the histogram can't resolve sub-$5 percentiles.

Fix

Add advice.explicitBucketBoundaries with cents-scale boundaries: [0.01, 0.05, 0.10, 0.25, 0.50, 1.00, 2.50, 5.00, 10.00, 25.00].

Evidence

ClickHouse bucket distribution against a live SigNoz (all 267 samples in le=5):

bucket | count
-------+------
0      | 0
5      | 267
10     | 267
25     | 267
...
inf    | 267

Bucket change affects new data only; existing histograms keep their old boundaries.

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Companion PR: #58

Summary by CodeRabbit

• New Features
  • Enhanced session cost tracking with improved cost bucket granularity for more precise reporting.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: baf719ba-adae-48a1-8601-6926809d5c01

📥 Commits

Reviewing files that changed from the base of the PR and between b5d4185 and 27edfaa.

📒 Files selected for processing (1)

• src/otel.ts

📝 Walkthrough

Walkthrough

The PR updates the observability telemetry configuration by adding explicit bucket boundaries to the sessionCostGauge histogram instrument. The histogram now publishes cost-per-session metrics using a predefined set of USD cost buckets (0.01 through 25.00) for improved metric granularity.

Changes

Session cost histogram configuration

Layer / File(s)	Summary
Configure explicit USD cost bucket boundaries src/otel.ts	The sessionCostGauge histogram instrument's advice.explicitBucketBoundaries is set with fixed USD cost thresholds (0.01–25.00) to define histogram bucket boundaries for cost-per-session metric export.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding cents-scale bucket boundaries to the session.cost.total histogram in the OpenTelemetry configuration.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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