USD values from CoinGecko for TokenBridge

.dialyzer-ignore

@@ -20,8 +20,8 @@ lib/block_scout_web/views/layout_view.ex:145: The call 'Elixir.Poison.Parser':'p
 lib/block_scout_web/views/layout_view.ex:237: The call 'Elixir.Poison.Parser':'parse!'
 lib/explorer/smart_contract/reader.ex:435
 lib/indexer/fetcher/token_total_supply_on_demand.ex:16
-lib/explorer/exchange_rates/source.ex:116
-lib/explorer/exchange_rates/source.ex:119
+lib/explorer/exchange_rates/source.ex:117
+lib/explorer/exchange_rates/source.ex:120
 lib/explorer/smart_contract/solidity/verifier.ex:223
 lib/block_scout_web/templates/address_contract/index.html.eex:158
 lib/block_scout_web/templates/address_contract/index.html.eex:195

apps/explorer/lib/explorer/chain/supply/token_bridge.ex

@@ -92,10 +92,8 @@ defmodule Explorer.Chain.Supply.TokenBridge do
   def total_market_cap_from_omni_bridge, do: Bridge.fetch_omni_bridge_market_cap()
 
   def total_chain_supply do
-    source = Application.get_env(:explorer, Source)[:source]
-
     usd_value =
-      case Source.fetch_exchange_rates(source) do
+      case Source.fetch_exchange_rates(Source.CoinGecko) do
         {:ok, [rates]} ->
           rates.usd_value
 

apps/explorer/lib/explorer/exchange_rates/source.ex

@@ -3,6 +3,7 @@ defmodule Explorer.ExchangeRates.Source do
   Behaviour for fetching exchange rates from external sources.
   """
   alias Explorer.ExchangeRates.{Source, Token}
+  alias Explorer.ExchangeRates.Source.CoinGecko
   alias HTTPoison.{Error, Response}
 
   @doc """
@@ -16,16 +17,16 @@ defmodule Explorer.ExchangeRates.Source do
 
   @spec fetch_exchange_rates_for_token(String.t()) :: {:ok, [Token.t()]} | {:error, any}
   def fetch_exchange_rates_for_token(symbol) do
-    source = Application.get_env(:explorer, Source)[:source]
-    source_url = source.source_url(symbol)
-    fetch_exchange_rates_request(source, source_url, source.headers())
+    source_url = CoinGecko.source_url(symbol)
+    headers = CoinGecko.headers()
+    fetch_exchange_rates_request(CoinGecko, source_url, headers)
   end
 
   @spec fetch_exchange_rates_for_token_address(String.t()) :: {:ok, [Token.t()]} | {:error, any}
   def fetch_exchange_rates_for_token_address(address_hash) do
-    source = Application.get_env(:explorer, Source)[:source]
-    source_url = source.source_url(address_hash)
-    fetch_exchange_rates_request(source, source_url, source.headers())
+    source_url = CoinGecko.source_url(address_hash)
+    headers = CoinGecko.headers()
+    fetch_exchange_rates_request(CoinGecko, source_url, headers)
   end
 
   defp fetch_exchange_rates_request(_source, source_url, _headers) when is_nil(source_url),