Kibana Platform creates two HTTP services

[jportner]

Kibana version: 7.6 RC+

Describe the bug: The Kibana Platform creates two HTTP services (one for setup, one for start). As a result, HTTP request/response interceptors in the Kibana Platform don't always get triggered.

Steps to reproduce: To quickly verify this behavior, in src/core/public/http/fetch.ts, add the following line at the beginning of the fetch method:

console.log(`fetch (interceptors: ${this.interceptors.size})`);

Then, log into Kibana and view the console to see fetch requests going through both HTTP services. Example:

Any additional context: It appears that a change to http_service.ts in #53033 inadvertently resulted in the plugin setup and start methods each receiving a different version of the HTTP service. If interceptors were registered during setup, they would not be affected by requests that were made with the HTTP service from start (and vice versa). Special thanks to @legrego for doing the heavy lifting in troubleshooting!

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)