feat(plugins/opentelemetry): Replace PeriodicReader with direct gRPC export

[ValentinCB]

Description

A output plugin which exports Alumet metrics to an OpenTelemetry collector via gRPC.

This PR replaces the previous opentelemetry-sdk-based implementation that used a PeriodicReader, which capped the export frequency at 1Hz regardless of Alumet's pipeline cadence. The new implementation calls the OTLP gRPC endpoint directly inside write(), allowing metrics to be exported at any frequency.

Functional tests are also added to cover the main export behaviors.

For more info, please refer to ./plugins/opentelemetry/README.md.

How to test

At the root of the repo, edit the alumet-config.toml file. Lower the pool intervals of RAPL and Procfs under 1s, and add the [opentelemetry] config field:

[plugins.opentelemetry]
# URL of the OTLP gRPC collector
collector_host = "http://localhost:4317"
# Optional prefix and suffix for metric names
prefix = ""
suffix = ""
# Use the display name for units (e.g. "gCO₂") instead of the unique name (e.g. "g_CO2")
use_unit_display_name = false
# Forward measurement attributes as OTel data-point attributes
add_attributes_to_labels = false

Start a local OpenTelemetry collector (e.g. with Docker):

docker run -p 4317:4317 otel/opentelemetry-collector

Build the binary and launch Alumet with rapl and opentelemetry:

cargo build
./target/debug/alumet-agent --config alumet-config.toml

You should see metrics being exported to your collector in real time.

To run the functional tests:

cargo test --test output -p plugin-opentelemetry

[guigomcha]

Super nice, just a few questions

[guigomcha]

LGTM! nice improvement!