API versioning

Since we're not a library it is required to notify our clients about API change.

For example a monitoring server API provides `/metrics` method, which returns a JSON. To be able to keep the proxy backward compatible we can add `/vN` prefix and it becomes `/v1/metrics`.

Moreover we should mention how we treat semantic versioning. Strictly, we should bump major version when:

- The config changes in backward-incompatible manner.
- We break URIs.

So, the tasks are:

- [ ] Wrap API paths with version prefix.
- [ ] Update README.md.

Start by locating the proxy's API route definitions and the `/metrics` handler, then read README.md for the documented API and versioning conventions. Add a version prefix to API paths while preserving the intended proxy compatibility, and update README.md to explain the prefix and when a major version is required.

Written by the indexing model from the issue text.