How to merge default tags with user provided tags in Terraform HCL

default_tags.tf

# Define a variable for user-provided tags as a map of strings.
variable "user_tags" {
  type        = map(string)
  default     = {}
  description = "A user defined map of tags that should be attached to every tagable resource."
}

locals {
  # Define the common tags that are added to all resources.
  common_tags = {
    "Managed by"       = "Terraform"
    "Terraform module" = path.module
  }

  # Merge `local.common_tags` with `var.user_tags` in a new local.
  # `var.custom_tags` takes precedence.
  all_tags = merge(
    local.common_tags,
    var.user_tags
  )
}

# Define a resource that allows tags.
resource "provider_resource" "placeholder" {
  # Resource configuration ...

  # Use `local.all_tags` in your module's resources.
  tags = local.all_tags
}