build: Upstream envoy's C++20 patch

Added in envoyproxy/envoy#32585.

Required for proxy-wasm/proxy-wasm-cpp-host#411

Signed-off-by: Matt Leon <mattleon@google.com>

Author: leonm1

proxy_wasm_api.h

@@ -1584,9 +1584,13 @@ template <> inline std::string toString(std::string t) { return t; }
 
 template <> inline std::string toString(bool t) { return t ? "true" : "false"; }
 
-template <typename T> struct StringToStringView { typedef T type; };
+template <typename T> struct StringToStringView {
+  typedef T type;
+};
 
-template <> struct StringToStringView<std::string> { typedef std::string_view type; };
+template <> struct StringToStringView<std::string> {
+  typedef std::string_view type;
+};
 
 inline uint32_t MetricBase::resolveFullName(const std::string &n) {
   auto it = metric_ids.find(n);
@@ -1712,9 +1716,9 @@ struct SimpleHistogram {
 template <typename... Tags> struct Counter : public MetricBase {
   static Counter<Tags...> *New(std::string_view name, MetricTagDescriptor<Tags>... fieldnames);
 
-  Counter<Tags...>(std::string_view name, MetricTagDescriptor<Tags>... descriptors)
-      : Counter<Tags...>(std::string(name), std::vector<MetricTag>({toMetricTag(descriptors)...})) {
-  }
+  template <typename... T>
+  Counter(std::string_view name, MetricTagDescriptor<T>... descriptors)
+      : Counter<T...>(std::string(name), std::vector<MetricTag>({toMetricTag(descriptors)...})) {}
 
   SimpleCounter resolve(Tags... f) {
     std::vector<std::string> fields{toString(f)...};
@@ -1763,8 +1767,9 @@ inline Counter<Tags...> *Counter<Tags...>::New(std::string_view name,
 template <typename... Tags> struct Gauge : public MetricBase {
   static Gauge<Tags...> *New(std::string_view name, MetricTagDescriptor<Tags>... fieldnames);
 
-  Gauge<Tags...>(std::string_view name, MetricTagDescriptor<Tags>... descriptors)
-      : Gauge<Tags...>(std::string(name), std::vector<MetricTag>({toMetricTag(descriptors)...})) {}
+  template <typename... T>
+  Gauge(std::string_view name, MetricTagDescriptor<T>... descriptors)
+      : Gauge<T...>(std::string(name), std::vector<MetricTag>({toMetricTag(descriptors)...})) {}
 
   SimpleGauge resolve(Tags... f) {
     std::vector<std::string> fields{toString(f)...};
@@ -1809,9 +1814,9 @@ inline Gauge<Tags...> *Gauge<Tags...>::New(std::string_view name,
 template <typename... Tags> struct Histogram : public MetricBase {
   static Histogram<Tags...> *New(std::string_view name, MetricTagDescriptor<Tags>... fieldnames);
 
-  Histogram<Tags...>(std::string_view name, MetricTagDescriptor<Tags>... descriptors)
-      : Histogram<Tags...>(std::string(name),
-                           std::vector<MetricTag>({toMetricTag(descriptors)...})) {}
+  template <typename... T>
+  Histogram(std::string_view name, MetricTagDescriptor<T>... descriptors)
+      : Histogram<T...>(std::string(name), std::vector<MetricTag>({toMetricTag(descriptors)...})) {}
 
   SimpleHistogram resolve(Tags... f) {
     std::vector<std::string> fields{toString(f)...};