Make types constants

Author: MrLotU

Sources/Prometheus/MetricTypes/Counter.swift

@@ -4,7 +4,7 @@ public class Counter<NumType: Numeric, Labels: MetricLabels>: Metric, Prometheus
     public let name: String
     public let help: String?
 
-    public var _type: MetricType = .counter
+    public let _type: MetricType = .counter
 
     internal var value: NumType
 

Sources/Prometheus/MetricTypes/Gauge.swift

@@ -4,7 +4,7 @@ public class Gauge<NumType: Numeric, Labels: MetricLabels>: Metric, PrometheusHa
     public let name: String
     public let help: String?
 
-    public var _type: MetricType = .gauge
+    public let _type: MetricType = .gauge
 
     private var value: NumType
 

Sources/Prometheus/MetricTypes/Histogram.swift

@@ -17,7 +17,7 @@ public class Histogram<NumType: DoubleRepresentable, Labels: HistogramLabels>: M
     public let name: String
     public let help: String?
 
-    public var _type: MetricType = .histogram
+    public let _type: MetricType = .histogram
 
     private var buckets: [Counter<NumType, EmptyCodable>] = []
 

Sources/Prometheus/MetricTypes/Summary.swift

@@ -17,7 +17,7 @@ public class Summary<NumType: DoubleRepresentable, Labels: SummaryLabels>: Metri
     public let name: String
     public let help: String?
 
-    public var _type: MetricType = .summary
+    public let _type: MetricType = .summary
 
     internal private(set) var labels: Labels