feat(terraform): monolithic support

[MichaelThamm]

Caution

Since we are making breaking changes:

1. API changes are not that serious because we can gate that on releases
2. juju_application reference changes are serious because this will replace the app. We need to either implement a move { } if we want the state to move. However, this is not too serious because our users use S3 storage backends anyways

Issue

• https://github.com/canonical/observability-team/issues/181

Solution

Summary

Replaces the three hardcoded worker modules (mimir_backend, mimir_read, mimir_write) with a single module.mimir_worker using for_each over a var.workers map. This enables both microservices and monolithic deployment modes from the same module interface.

Motivation

• Reduce duplication: all three worker modules were identical except for the role config key
• Enable monolithic mode (role-all) with a single worker instead of three
• Simplify the variable interface from ~15 per-role variables to one structured map

Usage

Microservices (default, same as before):

module "mimir" {
  source = "git::https://github.com/canonical/mimir-operators//terraform"
  # ...
  workers = {
    backend = { units = 3 }
    read    = { units = 2, storage_directives = { "data" = "10G" } }
    write   = { units = 2 }
  }
}

Monolithic (single worker with all roles):

module "mimir" {
  source = "git::https://github.com/canonical/mimir-operators//terraform"
  # ...
  workers = {
    all = { units = 3 }
  }
}

Breaking Changes

Before	After
var.backend_name / var.read_name / var.write_name	var.workers.<role>.app_name
var.backend_config / var.read_config / var.write_config	var.workers.<role>.config
var.backend_units / var.read_units / var.write_units	var.workers.<role>.units
var.backend_worker_storage_directives / etc.	var.workers.<role>.storage_directives
var.worker_constraints	var.workers.<role>.constraints
module.mimir_backend / module.mimir_read / module.mimir_write	module.mimir_worker["backend"] / ["read"] / ["write"]
juju_integration.coordinator_to_backend / etc.	juju_integration.coordinator_to_worker["backend"] / etc.

State migration: Existing deployments will need to recreate worker resources. The Terraform state addresses change due to the for_each refactor.

Validations

• Worker map keys restricted to ["all", "backend", "read", "write"]
• "all" cannot coexist with other role keys
• Per-worker units must be >= 1
• Anti-affinity (top-level bool) overrides per-worker constraints when enabled

Unchanged

• Coordinator module, S3 integrator, and their variables remain the same
• var.worker_resources and var.worker_revision remain top-level (shared across all workers)
• provides/requires outputs unchanged
• app_names output now dynamically includes whichever workers are deployed

Context

Testing Instructions

• Add unit tests to assert features work

Upgrade Notes