Open Source Prometheus Support for prometheus-sink

[ps48]

Description

The prometheus-sink plugin is currently hardcoded for AWS Managed Prometheus (AMP), making it impossible to use with open source Prometheus instances. Additionally, the README documents many features that are not actually implemented.

Issue 1: Open Source Prometheus (No Auth) Is Not Supported

The aws configuration block is annotated with @NotNull, requiring it for every pipeline. This means:

• HTTP URLs (http://) are rejected by the isValidConfig() validation — only https:// is allowed
• A SigV4 signer is always created, which fails without AWS credentials
• The x-amz-content-sha256 header is unconditionally added to every request, which vanilla Prometheus does not expect
• The Content-Encoding header is sent as "SNAPPY" (uppercase, from Java enum toString()) instead of "snappy" (lowercase), which Prometheus rejects

Expected Behavior

Users should be able to send metrics to an open source Prometheus Remote Write endpoint with a simple config like:

sink:
  - prometheus:
      url: "http://localhost:9090/api/v1/write"

Affected Files

• PrometheusSinkConfiguration.java — @NotNull on awsConfig, isValidConfig() rejects http://
• PrometheusHttpSender.java — unconditional x-amz-content-sha256 header, uppercase Content-Encoding

Issue 2: README Documents Features That Are Not Implemented

The current README (data-prepper-plugins/prometheus-sink/README.md) documents configuration options and usage examples for features that have config classes defined but are never wired up in the actual request path.

Documented Feature	Status
HTTP Basic authentication (authentication.http_basic)	Config classes exist, never instantiated
Bearer token / OAuth authentication (authentication.bearer_token)	Config classes exist, never instantiated
Proxy support (proxy)	Not implemented
Custom headers (custom_header)	Not implemented
HTTP method (http_method)	Not implemented (always POST)
SSL/TLS options (insecure, insecure_skip_verify, ssl_certificate_file, etc.)	Not implemented
DLQ file/plugin (dlq_file, dlq)	Not implemented
Buffer type (buffer_type)	Not implemented
sigv4 toggle in AWS config	Not implemented (SigV4 is always on when aws is present)

Note: The handler classes (BasicAuthPrometheusSinkHandler, BearerTokenAuthPrometheusSinkHandler, MultiAuthPrometheusSinkHandler) exist but use Apache HttpClient 5, while the actual sender (PrometheusHttpSender) uses Armeria. They are never called.

Expected Behavior

The README should only document features that are actually functional, with unimplemented features clearly marked or removed.

Environment

• Data Prepper version: 2.15 (main branch)
• Prometheus: v2.x / v3.x with --web.enable-remote-write-receiver