Add getLabelBasedMetricOwners API

[IvanIvanoff]

Changes

For all label-based metrics, the owner column holds the exchange name. This API is used to fetch the owners (exchanges) that a given metric is computed for.

The API also optionally accepts a slug parameter.

{
  getLabelBasedMetricOwners(metric: "exchange_open_interest")
}

{
  "data": {
    "getLabelBasedMetricOwners": [
      "binance",
      "bybit",
      "cryptodotcom",
      "okex",
      "kraken",
      "bitfinex",
      "btcex",
      "bitmex",
      "crosstower"
    ]
  }
}

Ticket

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have tried to find clearer solution before commenting hard-to-understand parts of code
• I have added tests that prove my fix is effective or that my feature works