[WFLY-18582] Add prometheus endpoint to micrometer extension

Add analysis document

observability/WFLY-18582_add-prometheus-endpoint-to-micrometer-extension.adoc

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+---
+categories:
+ - observability
+---
+= WFLY-18582 - Add prometheus endpoint to micrometer extension
+:author:            Jason Lee
+:email:             [email protected]
+:toc:               left
+:icons:             font
+:idprefix:
+:idseparator:       -
+
+== Overview
+
+Micrometer replaced Micrometer Metrics as the user-facing metrics library provided by WildFly. As part of that
+transition, the export behavior moved from a pull-based Prometheus-like format to push-based mechanism using the
+OpenTelemetry Protocol. Based on feedback from users who still desire to use the pull-based approach, this change will
+add support for the Prometheus Registry in addition to or in place of the OTLP Registry.
+
+== Issue Metadata
+
+=== Issue
+
+* https://issues.redhat.com/browse/WFLY-18582[WFLY-18582]
+
+=== Dev Contacts
+
+* mailto:{email}[{author}]
+
+=== QE Contacts
+
+=== Testing By
+* [x] Engineering
+
+* [ ] QE
+
+=== Affected Projects or Components
+
+Only the Micrometer subsystem and related tests will be affected
+
+=== Other Interested Projects
+
+=== Relevant Installation Types
+* [x] Traditional standalone server (unzipped or provisioned by Galleon)
+
+* [ ] Managed domain
+
+* [x] OpenShift s2i
+
+* [x] Bootable jar
+
+== Requirements
+
+=== Hard Requirements
+
+* Existing servers, if configured to use Micrometer (e.g., the `endpoint` property has been set), will be migrated to be
+and OTLP-based configuration. Since OTLP is the only current option, there is no ambiguity in this migration.
+* Existing servers with the extension enabled but with no endpoint set will not be changed. That is to say, Micrometer
+will continue to use a no-op registry.
+* Support for a new registry, `PrometheusRegistry` will be added.
+** It can be added as the only registry, or it can be added _in addition to_ the OTLP Registry
+** The only configuration option exposed for this registry is the web context (e.g., `/prometheus`) on which the metrics
+will be exposed
+** If the user would like to use `/metrics`, the WildFly metrics extension must be disabled first to avoid a naming conflict.
+
+=== Non-Requirements
+
+* Adding other arbitrary MeterRegistry types
+
+== Backwards Compatibility
+
+* Existing servers should continue to operate as is, using any existing OTLP configuration without manual intervention.
+
+=== Default Configuration
+
+* The default context for the Prometheus scrape will be undefined and must be specified by the user when enabling support (`context`)
+* The context will be secured (`secutiry-enabled`) by default
+
+=== Importing Existing Configuration
+
+* See above.
+
+=== Deployments
+
+=== Interoperability
+
+
+== Security Considerations
+
+The implementation of securing this endpoint will mirror how base metrics are secured.
+
+== Test Plan
+
+* Tests will be added to `testsuite/integration/microprofile/`
+
+== Community Documentation
+
+The WildFly Administration Guide will be updated to include this new support.
+
+== Release Note Content
+
+The Micrometer extension has been modified to add support for the Prometheus Registry, allowing for pull-based/scraping
+of metrics information for deployments where that is preferred.