OSS: `Gzip-Compression-Level` header parameter missing on `/backup/metadata`

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## Summary

The `Gzip-Compression-Level` request header parameter, added to the OSS
spec in influxdata/openapi#652 (commit influxdata/openapi@e48e733), is
applied only to the `GET /backup/shards/{shardID}` operation. It should
**also** be applied to `GET /backup/metadata`.

## Why this matters

The corresponding implementations in `influxd` and `influx-cli` set the
`Gzip-Compression-Level` header on **both** backup endpoints — so the
spec under-documents the feature.

- Server: `influxdata/influxdb#27297` adds the header on
`GET /api/v2/backup/shards/:shardID` **and**
`GET /api/v2/backup/metadata`.
- CLI: `influxdata/influx-cli#574` (`clients/backup/backup.go`) sets the
header on the metadata download request and on each shard download
request.

Without this fix, the rendered v2 API reference will show the new
compression option on the shard endpoint only, omitting it from the
metadata endpoint that uses the same flag from the CLI.

## Suggested change

In `src/oss/paths/backup_metadata.yml` (and any analogous Cloud paths
if the feature applies there), add the `Gzip-Compression-Level` header
parameter mirroring the block in `src/oss/paths/backup_shards_shardID.yml`:

```yaml
- in: header
name: Gzip-Compression-Level
description: The gzip compression level to use when compressing the response.
schema:
type: string
enum:
- none
- default
- speedy
- full
```

Once merged on `master`, please also promote to
`docs-release/influxdb-oss` so it lands in the published v2 API
reference.

## Context

Surfaced while documenting InfluxDB OSS 2.9.0 backup gzip compression
for influxdata/DAR#682. The DAR docs PR will reference the new
`--gzip-compression-level` flag and the HTTP header on both endpoints
in prose; this issue tracks the spec work needed for the API reference
to match.

cc @influxdata/edge

Compare src/oss/paths/backup_metadata.yml with src/oss/paths/backup_shards_shardID.yml, then inspect any analogous Cloud paths to determine whether the header applies there. Add the documented Gzip-Compression-Level parameter to the metadata operation and promote the change to docs-release/influxdb-oss so the published v2 API reference includes both endpoints.

Written by the indexing model from the issue text.