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The prometheus exporter is a Solr specific process for getting metrics from /admin/metrics and requires constant maintenance with any new metrics or big changes that Solr introduces. Also running this external process can be costly to maintain.

Proposed changes

OpenTelemetry

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SDK - Prometheus

Solr already uses Open Telemetry as a module for distributed tracing which integrates the SDK with auto-configuration. We aim to leverage the Open Telemetry SDK and move its dependencies into core. 

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• The API is used to measure/capture metrics with instruments in Solr. Open Telemetry supports different types of instruments which we will try to migrate from Dropwizard's equivalent. See Open Telemetry Meter section from its documentation.
• The SDK is then used to configure and export the API measurements collected from Solr by providing a number of different exporters from OTLP, prometheus or even in-memory metric readers. If not the SDK, the Java agent can also be used as a “zero-code” option

In Solr core, we will implement the Open Telemetry API to collect and record metrics.

We will then add only the Open Telemetry Prometheus Exporter SDK into core to expose these metrics to the /admin/metrics endpoint as prometheus. This will work right out of the box for Solr meaning by default Solr's exposed method for collecting metrics is through Prometheus and a pull based protocol.With the SDK, Solr can be used as both to expose both push and pull implementations for metrics. Push with OTLP which is becoming an industry standard with multiple different plugins and tools for exporting these and pull with prometheus as an endpoint to scrape from (/admin/metrics)

SolrMetricsContext needs to be completely refactored instead of being wrapped around Dropwizard to being wrapped around Open Telemetry. Some custom metric types that Solr uses from Dropwizard might not be possible or too difficult to port over to OTel such as MetricsMaps or complex GaugesTelemetry. Some custom metric types that Solr uses from Dropwizard might not be possible or too difficult to port over to OTel such as MetricsMaps or complex Gauges.

OpenTelemetry SDK - OTLP

For users who want to use collect Solr metrics with a push based protocol using OTLP, then they would need to enable the Open Telemetry OTLP module the same way the module is enabled with distributed trace. Push with OTLP which is becoming an industry standard with multiple different plugins and tools for exporting these metrics.

Metrics API /admin/metrics

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Users will now be open to the option of using Open Telemetry and its push model with OTLP metrics, similar to how traces and spans are being exported but must enable the Open Telemetry OTLP module.