[API] Parse baggage value as spec compliant (#3758)

Author: ThomsonTan

CHANGELOG.md

@@ -21,6 +21,9 @@ Increment the:
 * [CONFIGURATION] File configuration - rename tls properties
   [#3805](https://github.com/open-telemetry/opentelemetry-cpp/pull/3805)
 
+* [API] Parse baggage value as spec compliant
+  [#3758](https://github.com/open-telemetry/opentelemetry-cpp/pull/3758)
+
 Breaking changes:
 
 * [CONFIGURATION] File configuration - remove zipkin

api/include/opentelemetry/baggage/baggage.h

@@ -274,8 +274,13 @@ class OPENTELEMETRY_EXPORT Baggage
       {
         ret.push_back(' ');
       }
-      else if (std::isalnum(str[i]) || str[i] == '-' || str[i] == '_' || str[i] == '.' ||
-               str[i] == '~')
+      // See https://www.w3.org/TR/baggage/#definition
+      else if ((str[i] >= '!')      /* 21 */
+               && (str[i] <= '~')   /* 7E */
+               && (str[i] != '"')   /* 22 */
+               && (str[i] != ',')   /* 2C */
+               && (str[i] != ';')   /* 3B */
+               && (str[i] != '\\')) /* 5C */
       {
         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
       {",", {}, {}},
       {",=,", {}, {}},