@Ryan the text field search syntax has absolutely nothing to do with the GraphQL API and its features or better the lack thereof.
This whole API would make a lot more sense to me if you could do simple QBE like:
query {
customer (id:"gid://shopify/Customer/123")
{
metafields {
edges {
node {
id
key: "my_metafield"
}
}
}
}
}