How to inject metadata, after pages API?

Summary

I feel like all the pieces I need exist, I'm just not sure how to piece them together. What I want to do is after all my pages have been created within GraphQL (I'm using the filesystem plugin, but that shouldn't matter).

I then want to create a search index, and add the generated JSON to my metadata, for use on a search page later on. This means the index can be generated at build time, rather than on all page loads. My issue is that I can only do this before the pages API is generated.

Basic example

This works inside my components mounted hook, (using lunr.js for the search):

mounted() {
    const self = this;
    this.index = lunr(function () {
        this.ref('id');
        this.field('title');
        this.field('excerpt');
        this.metadataWhitelist = ['position'];

        self.allContent.forEach(post => {
            this.add(post.node);
        });
    });
}

I want to be able to do this within gridsome.server.js:

module.exports = function (api) {
    api.loadSource(async store => {
        // no data has been loaded for my pages at this point
        store.addMetaData('searchIndex', 'This is my search index');
    });

    api.beforeBuild(() => {
        // Load source isn't triggered at this point, and doesn't work in dev
        api.loadSource(async store => {
            store.addMetaData('searchIndex', 'This is my search index');
        });
    });
};

Motivation

To be able to inject metadata into the site based on page level data that has already been processed, such as a search index.

I have this exact same question. I tried referencing api._app.store.addMetaData but surprise, surprise! That extremely hacky solution doesn't work. So far I haven't been able to find any good option for generating static metadata based on GraphQL content.

I came across this usecase today. @hjvedvik any pointers on how this can be done? I need to create some information based on a GraphQL query and inject it as global metadata.

I'm in the same spot. I'm trying to use a custom resolver for the metadata field, but it's still unclear how to run a graphql query from inside the resolver.