Linkerd2: Document service discovery and load balancing

[briansmith]

At https://conduit.io/docs we don't have any documentation for how service discovery and load balancing works. It is important that we document how these work, in particular:

• How the proxy uses the Destination service to provide L7 load balancing.
• The advantages of how Conduit does L7 load balancing vs. how Kubernetes does L4 load balancing by default.
• How Conduit integrates the Destination service with Kubernetes load balancing transparently so that things like "clusterFirst" networking and custom dnsConfigs work transparently.
• How Conduit allows node-local services like "metadata" on GKE to work.
• Details of Conduit's load balancing, "least loaded", etc.
• Future enhancement plans.
• Limitations of our design, especially ways that things aren't fully transparent.
• How to override Conduit's service discovery enhancements.

I'm assigning this to myself to knock out a first draft.

[briansmith]

/cc @olix0r

[briansmith]

/cc @adleong

[c4milo]

Any updates or a non official doc that we can read in the mean time? Thanks!

[wmorgan]

Now in https://linkerd.io/2/features/load-balancing/