e2e_plan cannot plan MetricsQL: UserInput::validate rejects every FrontendInput::Metricsql #534

Problem

UserInput::validate (crates/planner/src/lib.rs, ~L87-104) accepts only these language/frontend pairs:

• QueryLanguage::SQL(_) or QueryLanguage::DataFusion with FrontendInput::Sql
• QueryLanguage::PromQL with FrontendInput::Promql

QueryLanguage (crates/types/src/workload.rs, ~L42) has the variants PromQL, SQL(_), DataFusion and ElasticDSL. None of them is MetricsQL. So any UserInput with FrontendInput::Metricsql fails validation with UserInputError::FrontendMismatch, and the FrontendInput::Metricsql branch of e2e_plan (~L253, lower_metricsql) is unreachable.

Before

Planning a MetricsQL workload through e2e_plan always fails before lowering, with FrontendMismatch { language: "PromQL", frontend: "Metricsql" } (or with whatever language is set).

Expected

A workload whose language is MetricsQL is accepted with FrontendInput::Metricsql, then lowered and planned.

Options

1. Add QueryLanguage::MetricsQL (serde "metricsql") and accept it with FrontendInput::Metricsql. Recommended: the workload then states its language explicitly. It is an additive workload-format change, so check whether any deployment matches on QueryLanguage exhaustively.
2. Accept QueryLanguage::PromQL with FrontendInput::Metricsql. Smaller, but it records MetricsQL workloads as PromQL.

Test

An e2e_plan test that plans one MetricsQL query end to end. It fails on main with FrontendMismatch.

Found while documenting extension scenarios for #533.