Check classic and OTel agents against serverless

[xrmx]

Description

Serverless requires to do authentication sending a base64 encoded API key, double check that it is working with the classic agent and with the OTEL_EXPORTER_OTLP_HEADERS environment variable in our OTel distributions.

Issues creation

Create Spec issue?

• Spec issue

Create APM Agent issues?

• elastic-otel-java
• elastic-otel-dotnet
• elastic-otel-node
• elastic-otel-python
• elastic-otel-php
• apm-agent-java
• apm-agent-dotnet
• apm-agent-nodejs
• apm-agent-python
• apm-agent-go
• apm-agent-php
• apm-agent-ruby