feat: add Redis identify ratelimiter

Myriad/Gateway/Limit/RedisRatelimiter.cs

@@ -0,0 +1,46 @@
+using Serilog;
+
+using StackExchange.Redis;
+
+namespace Myriad.Gateway.Limit;
+
+public class RedisRatelimiter: IGatewayRatelimiter
+{
+    private readonly ILogger _logger;
+    private readonly ConnectionMultiplexer _redis;
+
+    private int _concurrency { get; init; }
+
+    // todo: these might need to be tweaked a little
+    private static TimeSpan expiry = TimeSpan.FromSeconds(5);
+    private static TimeSpan retryInterval = TimeSpan.FromSeconds(1);
+
+    public RedisRatelimiter(ILogger logger, ConnectionMultiplexer redis, int concurrency)
+    {
+        _logger = logger.ForContext<TwilightGatewayRatelimiter>();
+        _redis = redis;
+        _concurrency = concurrency;
+    }
+
+    public async Task Identify(int shard)
+    {
+        _logger.Information("Shard {ShardId}: requesting identify from Redis", shard);
+        var key = "pluralkit:identify:" + (shard % _concurrency).ToString();
+        await AcquireLock(key);
+    }
+
+    public async Task AcquireLock(string key)
+    {
+        var conn = _redis.GetDatabase();
+
+        async Task<bool> TryAcquire()
+        {
+            _logger.Verbose("Trying to acquire lock on key {key} from Redis...", key);
+            await Task.Delay(retryInterval);
+            return await conn!.StringSetAsync(key, 0, expiry, When.NotExists);
+        }
+
+        var acquired = false;
+        while (!acquired) acquired = await TryAcquire();
+    }
+}