Support OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE, OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION, and OTEL_METRIC_EXPORT_INTERVAL environment variables

[pirgeo]

Description
When using the Micrometer OTLP exporter in a Spring Boot application, the OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE environment variable is ignored. This works as expected when running Micrometer standalone, but not when running via Spring Boot. Other environment variables, like OTEL_EXPORTER_OTLP_METRICS_ENDPOINT work fine.

Steps to Reproduce
Set the OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE environment variable (e.g., to DELTA).
Start a Spring Boot application with Micrometer OTLP metrics export enabled.
Observe that the temporality preference is not picked up from the environment variable.

Expected Behavior
The exporter should respect the OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE environment variable when not explicitly set in properties.

Actual Behavior
The environment variable is ignored, and the default value (CUMULATIVE) is always used.

Root Cause (Analysis)
The OtlpMetricsProperties#getAggregationTemporality() method returns a default value even when not set by the user, and never returns null. This prevents the fallback logic in OtlpMetricsPropertiesConfigAdapter from calling OtlpConfig.super::aggregationTemporality, which is where Micrometer reads the environment variable.

The problem also exists for OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION, probably due to the default value set in OtlpMetricsPropertiesConfigAdapter. Setting the environment variable OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION=base2_exponential_bucket_histogram seems to get ignored.
OTEL_METRIC_EXPORT_INTERVAL seems to have the same problem, but I don't know if it's due to the same reason.

Environment:
Spring Boot version: 3.5.6
Micrometer version: 1.15.4
Java version: 17
OS: macOS

[wilkinsona]

Two properties are relatively easy to address:

• aggregationTemporality
  • OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE
• histogramFlavor
  • OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION

The defaults for both of these are defined in OtlpMetricsProperties which is specific to OTLP.

A third is harder:

• step
  • OTEL_METRIC_EXPORT_INTERVAL

In this case, the default is defined in PushRegistryProperties which is shared by 10 different registries. We'll have to see what we can do there. Perhaps we can leave all of the defaults in place and use an environment post-processor to map the OTEL env vars to Boot configuration properties.

This feels like an enhancement to me, similar to #43712. Even if it were considered a bug, I don't think we could fix it in 3.5.x as it could result in an unwanted change in behavior.

[mhalbritter]

I've documented the supported environment variables for #47960. Please also add OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE, OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION and OTEL_METRIC_EXPORT_INTERVAL to the doc when this is implemented.