Implement Azure Functions metadata detection

[z1c0]

Description

See:

• https://github.com/elastic/apm/blob/main/specs/agents/metadata.md#azure-functions
  • Gherkin spec: https://github.com/elastic/apm/blob/main/tests/agents/gherkin-specs/azure_functions_metadata.feature
• https://github.com/elastic/apm/pull/712

Agent Issues

• [ ] https://github.com/elastic/apm-agent-java/issues/2964
• [x] https://github.com/elastic/apm-agent-dotnet/issues/1974
• [ ] https://github.com/elastic/apm-agent-nodejs/issues/3089
• [ ] https://github.com/elastic/apm-agent-python/issues/1722
• [ ] https://github.com/elastic/apm-agent-go/issues/1369
• [ ] https://github.com/elastic/apm-agent-php/issues/849
• [ ] https://github.com/elastic/apm-agent-ruby/issues/1348