--database-url startup flag should take precedence over HASURA_GRAPHQL_METADATA_DATABASE_URL env var

### Version Information

Server Version: 2.10.1

### What is the current behaviour?

Hasura requires a metadata database connection on startup.

The connection string for this database can can be provided in one of four ways, in order of precedence

- `--metadata-database-url` flag
- `HASURA_GRAPHQL_METADATA_DATABASE_URL` env var
- `--database-url` flag
- `HASURA_GRAPHQL_DATABASE_URL` env var

### What is the expected behaviour?

Flags should take precedence over env vars even in cases where redundant flags exist. Expected order of precedence:

- `--metadata-database-url` flag
- `--database-url` flag
- `HASURA_GRAPHQL_METADATA_DATABASE_URL` env var
- `HASURA_GRAPHQL_DATABASE_URL` env var

Start at the server startup configuration path that resolves --metadata-database-url, --database-url, HASURA_GRAPHQL_METADATA_DATABASE_URL, and HASURA_GRAPHQL_DATABASE_URL. Trace how these four inputs are prioritized and verify that the two flags precede both environment variables, with --metadata-database-url remaining highest priority.

Written by the indexing model from the issue text.