ProcessorForRequestHeader now looks first for a header like
`Content-Type: application/json; schema="prometheus/telemetry";
version="0.0.1"` before falling back to checking
`X-Prometheus-API-Version`.
client_golang was updated to support full label-oriented telemetry,
which introduced interface incompatibilities with the previous
version of Prometheus. To alleviate this, a general fetching and
processing dispatching system has been created, which discriminates
and processes according to the version of input.