Collect Azure App Service cloud metadata

Azure App Services are a very popular PaaS offering on Azure to run Web apps, Azure function apps, containerized web apps and mobile apps. In contrast to Azure VMs, App Services do not have access to the internal metadata endpoint in order to retrieve metadata about the app instance. Instead, much of the metadata that is relevant to the purposes of APM are available in environment variables.

APM agents should be able to collect corresponding environment variables to provide metadata on Azure App Services.

## Spec issue
- [x] https://github.com/elastic/apm/pull/365

## Agent issues
- [ ] https://github.com/elastic/apm-agent-java/issues/1578
- [x] https://github.com/elastic/apm-agent-dotnet/pull/1083
- [x] https://github.com/elastic/apm-agent-go/issues/863
- [ ] https://github.com/elastic/apm-agent-php/issues/312
- [x] https://github.com/elastic/apm-agent-ruby/issues/917
- [ ] https://github.com/elastic/apm-agent-nodejs/issues/1928
- [x] https://github.com/elastic/apm-agent-python/issues/990