Server-side validation - envvar interpolation about otelbin

[mmanciop]

We should detect browser-side env vars to be interpolated and, before validating backend-side, ask for test values to be used in the backend validation.

[mmanciop]

Based on open-telemetry/opentelemetry-collector#4742, when validating backend-side, in the frontend we need to:

1. Detect \${([a-zA-Z_]+:)?([^}]+)} and \$[a-zA-Z_][a-zA-Z0-9_]* sequences.

2. Check whether the first matching group in \${([a-zA-Z_]+:)?([^}]+)} is not the empty string, and therefore it is a configuration resolver prefix. If we do find a configuration resolver prefix, and it is env:, the second capture group denotes an environment variable name. Other prefixes, like file:, need to raise an error that disables backend validation, as we won't be able to resolve the values server-side.

Given the list of environment variable names found with the steps above, we need to prompt the user for input before any backend validation can occur. I do not see a meaningful and practical way of proposing default values at this stage.

As a note about other configuration providers (e.g., file:): while they will prevent any validation from succeeding, by disabling validation, we are giving up on catch other errors before we get stuck on the unresolvable config. However, the UX would be consistent, and it seems like the better trade-off.

[roshan-gh]

@mmanciop opened the issue