[bug] PeriodicExportingMetricReader missing validation for export_timeout_millis <= 0

Describe your environment

OS: macOS / Linux
Python version: 3.10+
SDK version: main

What happened?

In opentelemetry-sdk (opentelemetry.sdk.metrics.export.PeriodicExportingMetricReader), export_interval_millis validates that its value is positive and raises a ValueError if export_interval_millis <= 0.

However, export_timeout_millis lacks validation for non-positive or negative values (<= 0). Initializing PeriodicExportingMetricReader(ConsoleMetricExporter(), export_timeout_millis=-1) or export_timeout_millis=0 is accepted without raising a ValueError.

Since _ticker() continuously calls self.collect(timeout_millis=self._export_timeout_millis), an unvalidated non-positive timeout value is passed into metric collection on every tick.

Steps to Reproduce

from opentelemetry.sdk.metrics.export import (
ConsoleMetricExporter,
PeriodicExportingMetricReader,
)

1. export_interval_millis correctly validates <= 0:

try:
PeriodicExportingMetricReader(
ConsoleMetricExporter(), export_interval_millis=0
)
except ValueError as e:
print("export_interval_millis validation working:", e)

2. export_timeout_millis lacks validation:

reader = PeriodicExportingMetricReader(
ConsoleMetricExporter(), export_timeout_millis=-1
)
print("Accepted without ValueError! actual=", reader._export_timeout_millis)

Expected Result

Initializing PeriodicExportingMetricReader with export_timeout_millis <= 0 should raise a ValueError (e.g. f"timeout value {self._export_timeout_millis} is invalid and needs to be larger than zero."), consistent with export_interval_millis.

Actual Result

Accepted without raising a ValueError.

Additional context

Location: opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/export/__init__.py around lines 451-482.

I am opening a PR for this fix :))

Would you like to implement a fix?

Yes

Tip

_{React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding +1 or me too, to help us triage it. Learn more here.}

Read opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/export/init.py around lines 451-482, starting with PeriodicExportingMetricReader and its existing export_interval_millis validation. Reproduce the constructor calls from the issue for zero and negative export_timeout_millis, then verify that both are rejected consistently with the interval validation.