test(metrics): Add validation tests for GCS metrics (#4160)

* test: add GCS metrics tests for request count, latencies, and download bytes.

* chore: add license header and use constant for read type metric attribute

* refactor: Rename o-direct flag to experimental-o-direct (#4155)

* Rename o-direct to experimental-o-direct

This change renames the hidden flag `o-direct` to `experimental-o-direct` to clearly indicate its experimental status. It keeps the flag hidden.

Changes include:
- Renaming `file-system.o-direct` to `file-system.experimental-o-direct` in `cfg/params.yaml`.
- Updating `cfg/config.go` to reflect the field rename to `ExperimentalODirect`.
- Updating usage in `internal/fs/fs.go`.
- Updating tests in `cmd/root_test.go` to use the new flag name.
- Marking the flag as experimental in the usage string.

* Rename o-direct to experimental-o-direct

This change renames the hidden flag `o-direct` to `experimental-o-direct` to clearly indicate its experimental status. It keeps the flag hidden.

Changes include:
- Renaming `file-system.o-direct` to `file-system.experimental-o-direct` in `cfg/params.yaml`.
- Updating `cfg/config.go` to reflect the field rename to `ExperimentalODirect`.
- Updating usage in `internal/fs/fs.go`.
- Updating tests in `cmd/root_test.go` to use the new flag name.
- Marking the flag as experimental in the usage string.

* Rename o-direct to experimental-o-direct

This change renames the hidden flag `o-direct` to `experimental-o-direct` to clearly indicate its experimental status. It keeps the flag hidden.

Changes include:
- Renaming `file-system.o-direct` to `file-system.experimental-o-direct` in `cfg/params.yaml`.
- Updating `cfg/config.go` to reflect the field rename to `ExperimentalODirect`.
- Updating usage in `internal/fs/fs.go`.
- Updating tests in `cmd/root_test.go` to use the new flag name.
- Marking the flag as experimental in the usage string.

* Rename o-direct to experimental-o-direct

This change renames the hidden flag `o-direct` to `experimental-o-direct` to clearly indicate its experimental status. It keeps the flag hidden.

Changes include:
- Renaming `file-system.o-direct` to `file-system.experimental-o-direct` in `cfg/params.yaml`.
- Updating `cfg/config.go` to reflect the field rename to `ExperimentalODirect`.
- Updating usage in `internal/fs/fs.go`.
- Updating tests in `cmd/root_test.go` to use the new flag name.
- Marking the flag as experimental in the usage string.

* Rename o-direct to experimental-o-direct

This change renames the hidden flag `o-direct` to `experimental-o-direct` to clearly indicate its experimental status. It keeps the flag hidden.

Changes include:
- Renaming `file-system.o-direct` to `file-system.experimental-o-direct` in `cfg/params.yaml`.
- Updating `cfg/config.go` to reflect the field rename to `ExperimentalODirect`.
- Updating usage in `internal/fs/fs.go`.
- Updating tests in `cmd/root_test.go` to use the new flag name.
- Marking the flag as experimental in the usage string.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

* feat: add GCS metrics tests for request count, latency, download bytes, and file cache interactions.

* feat: add GCS metrics tests for request count, latency, download bytes, and file cache interactions.

* fix: Corrected error return in fake bucket manager and refined GCS metrics test descriptions and setup.

* fix: Corrected error return in fake bucket manager and refined GCS metrics test descriptions and setup.

* fix: Corrected error return in fake bucket manager and refined GCS metrics test descriptions and setup.

* style: format gcs_metrics_test.go by adjusting blank lines.

* Empty commit

* chore: update copyright year to 2026

* Empty commit

* fix: Update GCS metrics test descriptions and increment request count verification

* fix: Update GCS metrics test descriptions and increment request count verification

* fix: Update GCS metrics test descriptions and increment request count verification

* fix: Update GCS metrics test descriptions and increment request count verification

* fix: Update GCS metrics test descriptions and increment request count verification

* fix: Update GCS metrics test descriptions and increment request count verification

* fix: Update GCS metrics test descriptions and increment request count verification

* fix: Add verification for request count in GCS metrics test

* feat: Add `gcs/read_bytes_count` metric verification and `ExperimentalParallelDownloadsDefaultOn` configuration to GCS metrics tests.

* test: validate gcs/retry_count metric for various retry categories

* style: adjust whitespace in test file.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: alleaditya