[APM] Uploading a source map with ${foo} in the service_version param prevents APM Server from working

[axw]

Kibana version: 8.4.3

Original install method (e.g. download page, yum, from source, etc.): Elastic Cloud

Describe the bug:

When a source map is uploaded via Kibana with service_version set to ${VERSION} (e.g. due to a typo in shell variable expansion), then APM Server becomes disabled when running under Elastic Agent and Fleet.

Steps to reproduce:

1. Create an Elastic Cloud deployment
2. Upload a source map (https://www.elastic.co/guide/en/kibana/current/rum-sourcemap-api.html#rum-sourcemap-post) with service_version=${VERSION} (literally -- don't expand the variable)
3. APM Server will shortly after become unavailable

Expected behavior:

APM Server should remain available. Source mapping probably won't work, unless the service version is truly ${VERSION}.

Any additional context:

Fixing this easily will probably require elastic/elastic-agent#2177. There is a workaround noted in that issue, but will require some additional parsing.

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)