monitoring: fix alert_policy mime_type permadiff on imported policies (#25127)

[jbbqqf]

Summary

Replace default_value: "text/markdown" with default_from_api: true on google_monitoring_alert_policy.documentation.mime_type, so that imported alert policies (or policies whose API response omits mime_type) stop showing a permanent diff to add mime_type = "text/markdown".

Fixes hashicorp/terraform-provider-google#25127 — see hashicorp/terraform-provider-google#25127

Why

The OP and a second reporter (dale-c-anderson) both confirm the bug only triggers on imported alert policies. The plan after terraform import consistently shows:

~ documentation {
    + mime_type = "text/markdown"
      # (2 unchanged attributes hidden)
}

What is happening:

1. default_value: "text/markdown" in the mmv1 YAML lowers to Default: "text/markdown" on the Terraform schema (see resource_monitoring_alert_policy.go:1218).
2. terraform import calls Read, which calls the flatten path. For an alert policy whose documentation has only a subject (or only links), the API response often omits mime_type entirely. The flatten therefore stores nil/empty in state.
3. On the next plan, the schema's Default re-injects "text/markdown" because nothing is set, and the framework reports a diff against the empty state value. Hence the perpetual + mime_type = "text/markdown".

The reverse (creating from scratch) doesn't hit this because the API echoes the mime_type back during create, so state is populated.

default_from_api: true is the standard mmv1 idiom for "let the API decide": the schema becomes Optional + Computed (no schema-side Default), so when the API doesn't return mime_type the state stays empty and the planner doesn't try to fill it back in.

GCP API reference: https://cloud.google.com/monitoring/api/ref_v3/rest/v3/projects.alertPolicies#Documentation — mimeType is documented as defaulting to "text/markdown" server-side.

What changed

mmv1/products/monitoring/AlertPolicy.yaml | 2 +-

One YAML line changed: default_value: "text/markdown" → default_from_api: true.

Edge cases tested

#	Scenario	Expected	Verified by
1	Imported alert policy with documentation { subject = "x" } (no content)	API omits mime_type from the response. State stays empty. Plan: no diff.	Static — default_from_api: true produces Optional+Computed (and no Default); when state is empty and config is empty, no diff.
2	New alert policy with documentation { content = "x" mime_type = "text/markdown" }	API echoes mime_type = "text/markdown". State and config match → no diff.	Static — same flow as today, just without the redundant schema default.
3	New alert policy with documentation { content = "x" } (no mime_type)	API stores the server default "text/markdown". Read populates state. Subsequent plans see Optional+Computed field with state value matching API → no diff.	Static — server default behavior documented in the linked API ref.
4	User changes mime_type from "text/markdown" to a different value	Real change — should show diff and apply.	Static — Optional is unchanged; the planner still detects user-set value differences.

Test protocol

Test	Result	Notes
YAML lint / mmv1 generation	n/a in this PR	Will be exercised by mmv1 CI. The change is a 1-line attribute swap; both default_value and default_from_api are well-trodden fields.
Live BEFORE/AFTER smoke	not run	Reproducing the bug requires (a) creating an alert policy via gcloud or the console, (b) running terraform import, (c) running plan twice — that's 4 GCP operations gated on a separately-created resource and not friendly to the smoke harness's apply→destroy model. The fix is structurally complete: default_from_api: true is the documented idiom for exactly this scenario, and the same swap was the resolution to several similar permadiff issues in mmv1 history (e.g. beyondcorp, storagecontrol).

Resources

• Original issue: google_monitoring_alert_policy always wants to add mime_type to documentation (even after applying that exact change) hashicorp/terraform-provider-google#25127
• Related (closed but reopened-as-this): Google Monitoring Alert Policy is changed in every plan hashicorp/terraform-provider-google#20507
• API doc: https://cloud.google.com/monitoring/api/ref_v3/rest/v3/projects.alertPolicies#Documentation

Disclosure

This PR was implemented with assistance from Claude Code as part of a focused contribution batch. The diff is one YAML line; it was reviewed manually against the generated Go schema and the mmv1 doctrine (default_value injects a Default: clause that fights with API omissions, while default_from_api: true makes the schema Optional+Computed).