feat: service discovery by default

[kohidave]

This change enables service discovery by default for
all apps. Service discovery alows apps within an
environment to communicate with each other without
having to traverse the internet (traffic is routed
internally within the VPC).

To access a service via its service discovery endpoint,
you can make a request like:

  resp, _ := http.Get(fmt.Sprintf("http://%s.%s", appName, os.Getenv("ECS_APP_DISCOVERY_ENDPOINT"))

Or, with just the project name:

  resp, _ := http.Get(fmt.Sprintf("http://%s.%s.local", appName, projectName))

Or, without the string interpolation:

  // Making a request to app `api` in project `kudos`
  resp, _ := http.Get("http://api.kudos.local")

The same address works across environments, since the requests
are scoped to services within one environment.

Most of this change is making our E2E test apps little services so we
can test that they actually spin up and are able to preform HTTP
calls over their service discovery endpoints.

Why not just .local ?

A couple of reasons to go for the .{project}.local scheme, rather
than just .local or just .{project}.

The reason for not using just .local is just to reduce the chance
of a namespace collision if another project is using the same VPC.

The reason for not just going with .{project} is in case the
project name matches an existing TLD. That'd be confusing behavior.
The .local TLD doesn't seem to exist.

Why make this default for every service?

You can't add service discovery after the fact, unfortunately.
The only sensible thing is to just enable it by default. In the
future, we can offer a disable knob, but it's very cheap (less
than a dollar per month per env) - so isn't a huge burden for
customers who end up not using it.

Resolves #599

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[efekarakus]

Awesome yay, looks really good! just a few questions

[iamhopaul123]

👍 just a small question: why not use "${appName}.local", since the domain is only available inside the same environment?

[kohidave]

👍 just a small question: why not use "${appName}.local", since the domain is only available inside the same environment?

The reason is in case there's another service discovery namespace in that VPC with .local. If someone spun up a new project in that VPC (if we allowed that) - then the .local would collide.

[iamhopaul123]

👍 just a small question: why not use "${appName}.local", since the domain is only available inside the same environment?

The reason is in case there's another service discovery namespace in that VPC with .local. If someone spun up a new project in that VPC (if we allowed that) - then the .local would collide.

oh I see what you mean.