Set proper Content-type

[better0fdead]

For now, if i export metrics with json i get that output:

f.terekhin@f-terekhin tt % curl -s -I -X GET http://127.0.0.1:8082/metrics/json
HTTP/1.1 200 Ok
Content-length: 17348
Server: Tarantool http (tarantool v3.2.0-entrypoint-0-g7fd530f)
Content-type: text/plain; charset=utf-8
Connection: keep-alive

Content-type is text/plain; charset=utf-8, not json.
Need to set proper Content-type.