[API] Parse baggage value as spec compliant

CHANGELOG.md

@@ -15,6 +15,9 @@ Increment the:
 
 ## [Unreleased]
 
+* [API] Parse baggage value as spec compliant
+  [#3758](https://github.com/open-telemetry/opentelemetry-cpp/pull/3758)
+
 ## [1.24 2025-11-20]
 
 * [RELEASE] Bump main branch to 1.24-dev

api/include/opentelemetry/baggage/baggage.h

@@ -274,8 +274,8 @@ class OPENTELEMETRY_EXPORT Baggage
       {
         ret.push_back(' ');
       }
-      else if (std::isalnum(str[i]) || str[i] == '-' || str[i] == '_' || str[i] == '.' ||
-               str[i] == '~')
+      else if (str[i] >= ' ' && str[i] <= '~' && str[i] != '"' && str[i] != ',' && str[i] != ';' &&
+               str[i] != '\\')
       {
         ret.push_back(str[i]);
       }

api/test/baggage/baggage_test.cc

@@ -71,6 +71,8 @@ TEST(BaggageTest, ValidateExtractHeader)
       {"1a-2f%40foo=bar%251,a%2A%2Ffoo-_%2Fbar=bar+4",
        {"1a-2f@foo", "a*/foo-_/bar"},
        {"bar%1", "bar 4"}},                                       // decoding is done properly
+      {"field=foo:bar", {"field"}, {"foo:bar"}},                  // colon in value
+      {"mixed=a/b:c?d=e", {"mixed"}, {"a/b:c?d=e"}},              // mixed special characters
       {"k1=v1,invalidmember,k2=v2", {"k1", "k2"}, {"v1", "v2"}},  // invalid member is skipped
       {",", {}, {}},
       {",=,", {}, {}},