Add RateLimitPrecision enum, set X-RateLimit-Precision

Adds the RateLimitPrecision enum, with Second and Millisecond values. (Do we want to use an extension method to convert it into a string, or is ToString().ToLower() fine?)
Adds RateLimitPrecision as a parameter to DiscordRestApiClient, and to DiscordConfig so that it can set the X-RateLimit-Precision header.

src/Discord.Net.Core/DiscordConfig.cs

@@ -36,7 +36,7 @@ namespace Discord
             typeof(DiscordConfig).GetTypeInfo().Assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion ??
             typeof(DiscordConfig).GetTypeInfo().Assembly.GetName().Version.ToString(3) ??
             "Unknown";
-        
+
         /// <summary>
         ///     Gets the user agent that Discord.Net uses in its clients.
         /// </summary>
@@ -123,7 +123,7 @@ namespace Discord
         ///     The currently set <see cref="RetryMode"/>.
         /// </returns>
         public RetryMode DefaultRetryMode { get; set; } = RetryMode.AlwaysRetry;
-        
+
         /// <summary>
         ///     Gets or sets the minimum log level severity that will be sent to the Log event.
         /// </summary>
@@ -140,5 +140,17 @@ namespace Discord
         ///     the API version it uses on startup.
         /// </remarks>
         internal bool DisplayInitialLog { get; set; } = true;
+
+        /// <summary>
+        ///     Gets or sets the level of precision of the rate limit reset response.
+        /// </summary>
+        /// <remarks>
+        ///     If set to <see cref="RateLimitPrecision.Second"/>, this value will be rounded up to the
+        ///     nearest second.
+        /// </remarks>
+        /// <returns>
+        ///     The currently set <see cref="RateLimitPrecision"/>.
+        /// </returns>
+        public RateLimitPrecision RateLimitPrecision { get; set; } = RateLimitPrecision.Second;
     }
 }

src/Discord.Net.Core/RateLimitPrecision.cs

@@ -0,0 +1,18 @@
+namespace Discord
+{
+    /// <summary>
+    ///     Specifies the level of precision to request in the rate limit
+    ///     response header.
+    /// </summary>
+    public enum RateLimitPrecision
+    {
+        /// <summary>
+        ///     Specifies precision rounded up to the nearest whole second
+        /// </summary>
+        Second,
+        /// <summary>
+        ///     Specifies precision rounded to the nearest millisecond.
+        /// </summary>
+        Millisecond
+    }
+}

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -45,17 +45,19 @@ namespace Discord.API
         internal string AuthToken { get; private set; }
         internal IRestClient RestClient { get; private set; }
         internal ulong? CurrentUserId { get; set; }
-
+        public RateLimitPrecision RateLimitPrecision { get; private set; }
+        
         internal JsonSerializer Serializer => _serializer;
 
         /// <exception cref="ArgumentException">Unknown OAuth token type.</exception>
         public DiscordRestApiClient(RestClientProvider restClientProvider, string userAgent, RetryMode defaultRetryMode = RetryMode.AlwaysRetry,
-            JsonSerializer serializer = null)
+            JsonSerializer serializer = null, RateLimitPrecision rateLimitPrecision = RateLimitPrecision.Second)
         {
             _restClientProvider = restClientProvider;
             UserAgent = userAgent;
             DefaultRetryMode = defaultRetryMode;
             _serializer = serializer ?? new JsonSerializer { ContractResolver = new DiscordContractResolver() };
+            RateLimitPrecision = rateLimitPrecision;
 
             RequestQueue = new RequestQueue();
             _stateLock = new SemaphoreSlim(1, 1);
@@ -71,6 +73,7 @@ namespace Discord.API
             RestClient.SetHeader("accept", "*/*");
             RestClient.SetHeader("user-agent", UserAgent);
             RestClient.SetHeader("authorization", GetPrefixedToken(AuthTokenType, AuthToken));
+            RestClient.SetHeader("X-RateLimit-Precision", RateLimitPrecision.ToString().ToLower());
         }
         /// <exception cref="ArgumentException">Unknown OAuth token type.</exception>
         internal static string GetPrefixedToken(TokenType tokenType, string token)

src/Discord.Net.WebSocket/BaseSocketClient.cs

@@ -80,7 +80,8 @@ namespace Discord.WebSocket
         internal BaseSocketClient(DiscordSocketConfig config, DiscordRestApiClient client)
             : base(config, client) => BaseConfig = config;
         private static DiscordSocketApiClient CreateApiClient(DiscordSocketConfig config)
-            => new DiscordSocketApiClient(config.RestClientProvider, config.WebSocketProvider, DiscordRestConfig.UserAgent);
+            => new DiscordSocketApiClient(config.RestClientProvider, config.WebSocketProvider, DiscordRestConfig.UserAgent,
+                rateLimitPrecision: config.RateLimitPrecision);
 
         /// <summary>
         ///     Gets a Discord application information for the logged-in user.

src/Discord.Net.WebSocket/DiscordShardedClient.cs

@@ -85,7 +85,8 @@ namespace Discord.WebSocket
             }
         }
         private static API.DiscordSocketApiClient CreateApiClient(DiscordSocketConfig config)
-            => new API.DiscordSocketApiClient(config.RestClientProvider, config.WebSocketProvider, DiscordRestConfig.UserAgent);
+            => new API.DiscordSocketApiClient(config.RestClientProvider, config.WebSocketProvider, DiscordRestConfig.UserAgent,
+                rateLimitPrecision: config.RateLimitPrecision);
 
         internal override async Task OnLoginAsync(TokenType tokenType, string token)
         {

src/Discord.Net.WebSocket/DiscordSocketApiClient.cs

@@ -38,8 +38,9 @@ namespace Discord.API
         public ConnectionState ConnectionState { get; private set; }
 
         public DiscordSocketApiClient(RestClientProvider restClientProvider, WebSocketProvider webSocketProvider, string userAgent,
-            string url = null, RetryMode defaultRetryMode = RetryMode.AlwaysRetry, JsonSerializer serializer = null)
-            : base(restClientProvider, userAgent, defaultRetryMode, serializer)
+            string url = null, RetryMode defaultRetryMode = RetryMode.AlwaysRetry, JsonSerializer serializer = null,
+            RateLimitPrecision rateLimitPrecision = RateLimitPrecision.Second)
+            : base(restClientProvider, userAgent, defaultRetryMode, serializer, rateLimitPrecision)
         {
             _gatewayUrl = url;
             if (url != null)

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -176,7 +176,8 @@ namespace Discord.WebSocket
             _largeGuilds = new ConcurrentQueue<ulong>();
         }
         private static API.DiscordSocketApiClient CreateApiClient(DiscordSocketConfig config)
-            => new API.DiscordSocketApiClient(config.RestClientProvider, config.WebSocketProvider, DiscordRestConfig.UserAgent, config.GatewayHost);
+            => new API.DiscordSocketApiClient(config.RestClientProvider, config.WebSocketProvider, DiscordRestConfig.UserAgent, config.GatewayHost,
+                rateLimitPrecision: config.RateLimitPrecision);
         /// <inheritdoc />
         internal override void Dispose(bool disposing)
         {