perf: replace the regex in MeterId with a byte loop

libs/meter/meter_id/meter_id.cpp

@@ -5,8 +5,16 @@
 
 namespace spectator {
 
-// Define the static member
-const std::regex INVALID_CHARS("[^-._A-Za-z0-9~^]");
+namespace {
+
+// The characters spectatord accepts in names and tags: [-._A-Za-z0-9~^]
+bool IsValidChar(const char c)
+{
+    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '-' || c == '.' ||
+           c == '_' || c == '~' || c == '^';
+}
+
+}  // namespace
 
 std::unordered_map<std::string, std::string> ValidateTags(const std::unordered_map<std::string, std::string>& tags)
 {
@@ -22,20 +30,36 @@ std::unordered_map<std::string, std::string> ValidateTags(const std::unordered_m
     return validTags;
 }
 
-std::string ReplaceInvalidChars(const std::string& s) { return std::regex_replace(s, INVALID_CHARS, "_"); }
+// Appends s with each invalid byte replaced by '_'. A byte loop rather than std::regex_replace, which was the dominant
+// cost of creating a MeterId.
+void AppendValidChars(std::string& out, const std::string& s)
+{
+    for (const char c : s)
+    {
+        out += IsValidChar(c) ? c : '_';
+    }
+}
 
 std::string ToSpectatorId(const std::string& name, const std::unordered_map<std::string, std::string>& tags)
 {
-    std::ostringstream ss;
-    ss << ReplaceInvalidChars(name);
-    if (!tags.empty())
+    // Each byte is kept or replaced by one byte, so the size is known up front and the id is allocated once.
+    std::size_t size = name.size();
+    for (const auto& [fst, snd] : tags)
     {
-        for (const auto& [fst, snd] : tags)
-        {
-            ss << "," << ReplaceInvalidChars(fst) << "=" << ReplaceInvalidChars(snd);
-        }
+        size += 1 + fst.size() + 1 + snd.size();
     }
-    return ss.str();
+
+    std::string id;
+    id.reserve(size);
+    AppendValidChars(id, name);
+    for (const auto& [fst, snd] : tags)
+    {
+        id += ',';
+        AppendValidChars(id, fst);
+        id += '=';
+        AppendValidChars(id, snd);
+    }
+    return id;
 }
 
 MeterId::MeterId(const std::string& name, const std::unordered_map<std::string, std::string>& tags)

libs/meter/meter_id/test_meter_id.cpp

@@ -31,6 +31,24 @@ TEST(MeterIdTest, IllegalCharsAreReplaced)
     EXPECT_EQ("test______^____-_~______________.___foo", id.GetSpectatordId());
 }
 
+TEST(MeterIdTest, IllegalCharsInTagsAreReplaced)
+{
+    const MeterId id("foo", {{"a b", "c/d"}});
+    EXPECT_EQ("foo,a_b=c_d", id.GetSpectatordId());
+}
+
+TEST(MeterIdTest, ValidCharsAreKept)
+{
+    const MeterId id("azAZ09-._~^");
+    EXPECT_EQ("azAZ09-._~^", id.GetSpectatordId());
+}
+
+TEST(MeterIdTest, EachNonAsciiByteIsReplaced)
+{
+    const MeterId id("caf\xc3\xa9");
+    EXPECT_EQ("caf__", id.GetSpectatordId());
+}
+
 TEST(MeterIdTest, LookupTags)
 {
     const MeterId id1("foo", {{"a", "1"}, {"b", "2"}, {"c", "3"}});