Proxy: solve event tracing with compile time strings.

This change creates trace events with a single parameter
composed of ClassName::Method.

The change additionally causes the duration of the proxy call to be
traced, not only the occurrence.

Fixed: webrtc:12787
Change-Id: I1689862318d4c6fc1dcef343c3ccf3ae9f7e17df
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/219788
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34149}

rtc_base/string_utils.h

@@ -88,6 +88,43 @@ std::string string_trim(const std::string& s);
 // TODO(jonasolsson): replace with absl::Hex when that becomes available.
 std::string ToHex(const int i);
 
+// CompileTimeString comprises of a string-like object which can be used as a
+// regular const char* in compile time and supports concatenation. Useful for
+// concatenating constexpr strings in for example macro declarations.
+namespace rtc_base_string_utils_internal {
+template <int NPlus1>
+struct CompileTimeString {
+  char string[NPlus1] = {0};
+  constexpr CompileTimeString() = default;
+  template <int MPlus1>
+  explicit constexpr CompileTimeString(const char (&chars)[MPlus1]) {
+    char* chars_pointer = string;
+    for (auto c : chars)
+      *chars_pointer++ = c;
+  }
+  template <int MPlus1>
+  constexpr auto Concat(CompileTimeString<MPlus1> b) {
+    CompileTimeString<NPlus1 + MPlus1 - 1> result;
+    char* chars_pointer = result.string;
+    for (auto c : string)
+      *chars_pointer++ = c;
+    chars_pointer = result.string + NPlus1 - 1;
+    for (auto c : b.string)
+      *chars_pointer++ = c;
+    result.string[NPlus1 + MPlus1 - 2] = 0;
+    return result;
+  }
+  constexpr operator const char*() { return string; }
+};
+}  // namespace rtc_base_string_utils_internal
+
+// Makes a constexpr CompileTimeString<X> without having to specify X
+// explicitly.
+template <int N>
+constexpr auto MakeCompileTimeString(const char (&a)[N]) {
+  return rtc_base_string_utils_internal::CompileTimeString<N>(a);
+}
+
 }  // namespace rtc
 
 #endif  // RTC_BASE_STRING_UTILS_H_