compute: add filter argument to google_compute_forwarding_rules data source (#20052)

[jbbqqf]

Summary

Adds a filter argument to the google_compute_forwarding_rules data
source, mirroring the filter query parameter supported by the
underlying compute.forwardingRules.list REST API. Without it, users
who want to enumerate forwarding rules for a specific label, network,
target type, or load-balancing scheme have to fetch every rule in the
region and filter in HCL with for_each + tomap workarounds.

Fixes hashicorp/terraform-provider-google#20052 — see hashicorp/terraform-provider-google#20052

Why

The Compute Engine ForwardingRules.list API exposes a filter query
parameter that uses the documented Google filter syntax
(labels.env:prod, network eq ".*/my-vpc", etc.). The vendored Go
client has had (*ForwardingRulesListCall).Filter(string) for years
(see google.golang.org/api/compute/v1/compute2-gen.go), but the
Terraform data source schema only exposed project / region / rules
— so users couldn't take advantage of server-side filtering, and large
GCP projects (hundreds of forwarding rules) saw both unnecessary API
load and inefficient state plans.

GCP API reference:

• https://cloud.google.com/compute/docs/reference/rest/v1/forwardingRules/list
• https://cloud.google.com/sdk/gcloud/reference/topic/filters (general filter expression syntax)

What changed

This change is to a hand-written Terraform data source (whose source
of truth lives in magic-modules mmv1/third_party/...). Files touched:

 .../data_source_google_compute_forwarding_rules.go | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

Three small edits in
mmv1/third_party/terraform/services/compute/data_source_google_compute_forwarding_rules.go:

1. Added a filter Optional TypeString schema entry with a
   description pointing to the API docs.
2. In the Read function, retrieved the user-supplied filter, included
   it in the data source ID (so distinct filter expressions produce
   distinct IDs), and applied it via listCall.Filter(filter) only
   when non-empty.
3. Set the resolved filter back into state at the end of Read (mirrors
   the existing pattern for project / region).

Edge cases tested

#	Scenario	HCL excerpt	Expected	Verified by
1	filter unset (default)	# filter omitted	Unchanged behavior — ID is projects/.../regions/.../forwardingRules, every rule in the region returned	Static: the if filter != "" guard skips both the ID suffix and the Filter() call
2	filter = "labels.env:prod"	filter = "labels.env:prod"	Server-side filtered list	Vendored client has (*ForwardingRulesListCall).Filter(string); the data source now wires it
3	filter with regex syntax	filter = "name eq \".*-prod-.*\""	Server-side regex filter	Same path as scenario 2; the API accepts the documented filter syntax verbatim
4	Two data-source instances with different filters	one with filter=...env:prod, one with filter=...env:dev	Distinct IDs, no state collision	The ID now includes /filter=<expr> when set

Test protocol

Test	Result	Notes
cd mmv1 && go run . --output ... --version ga --no-docs	OK	Regeneration completed cleanly
go build ./google/services/compute/... (TPG)	OK	full compute service compiles
go vet ./google/services/compute/... (TPG)	OK	only pre-existing unreachable-code warnings in *_firewall_policy_with_rules.go (unchanged by this PR)
Generated source contains filter schema entry and Filter(filter) call	OK	grep'd data_source_google_compute_forwarding_rules.go after regen — confirmed

This is purely additive (data source only — no resource lifecycle
implications): omitting filter reproduces the exact previous
behavior. The only risk surface is the server's reaction to an invalid
filter expression, and that surface is identical to what
(*ForwardingRulesListCall).Filter(...) already provides — i.e. the
caller gets the same 400 Bad Request they'd get from gcloud compute forwarding-rules list --filter=....

Resources

• Issue: Add filter argument to forwarding_rules data source hashicorp/terraform-provider-google#20052
• API method (with filter param):
  https://cloud.google.com/compute/docs/reference/rest/v1/forwardingRules/list
• mmv1 source file:
  https://github.com/jbbqqf/magic-modules/blob/feat/20052-forwarding-rules-filter/mmv1/third_party/terraform/services/compute/data_source_google_compute_forwarding_rules.go

Release notes

compute: added `filter` argument to the `google_compute_forwarding_rules` data source.

Disclosure

This PR was implemented with assistance from Claude Code. The diff was
modeled on the existing filter patterns used by other data sources
(e.g. google_compute_instances) and the vendored Go client's
Filter() builder. The regenerated provider compiled and vetted
cleanly. The author (a human) reviewed the diff and the regenerated
data source before opening this PR.

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