oss_19_periodic_metric_reader_batch_size

[ocelotl]

Closes #21

Add optional max_export_batch_size to PeriodicExportingMetricReader with data-point chunking under the export lock (Linear OSS-19, finding M4); None = unchanged single export. Validation: metrics suite 309 passed, scope sdk+changelog.

Linear issue: https://linear.app/dash0/issue/OSS-19/m4-periodicexportingmetricreader-lacks-max-export-batch-size

[ocelotl]

🔒 Internal (dash0) — not for upstream.

• Linear: https://linear.app/dash0/issue/OSS-19/m4-periodicexportingmetricreader-lacks-max-export-batch-size (finding M4)
• Closes: oss_19_periodic_metric_reader_batch_size #21 · Branch: oss_19_periodic_metric_reader_batch_size · Commit: 0fec0b5c8
• Validation: full metrics suite 309 passed; scope sdk + changelog.

[ocelotl]

📣 Public-facing draft — to be used for the upstream PR in open-telemetry/opentelemetry-python. No internal references; copy verbatim.

Title: Add max_export_batch_size to PeriodicExportingMetricReader

What

Adds an optional max_export_batch_size: int | None = None to PeriodicExportingMetricReader.

• None (default): unchanged — the full MetricsData is exported in a single export() call.
• Positive integer: the collected metrics are split into multiple MetricsData batches so no single export() call carries more than that many data points. Each batch reproduces the owning resource and scope, so grouping is preserved and no point is dropped or duplicated. A single metric is never split across the metric boundary; if one metric's own point count exceeds the limit it is emitted whole in its own batch. Batching runs under the existing _export_lock, so export() is still never concurrent.
• A value <= 0 raises ValueError in the constructor.

Why

The metrics SDK spec defines maxExportBatchSize for the periodic reader and Go implements it (sdk/metric/periodic_reader.go). Python had no way to bound points per export call, which can produce oversized payloads on large cycles.

Spec

Implements the periodic reader maxExportBatchSize from specification/metrics/sdk.md.

Tests

New tests: None/default → one export with all points; small batch size → multiple export calls each within the limit, names present once in order, resource/scope preserved; batch size larger than total → single call; direct _batch_metrics_data coverage on multi-resource/scope/metric input; invalid values (0, -1) → ValueError. All existing metrics tests pass (309).