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

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:

- 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.