[log] Do not set absl min log level if user does not set GRPC_VERBOSITY (#36931)

Do not set `absl::MinLogLevel` if user does not set `GRPC_VERBOSITY`. Only set `absl::MinLogLevel` if the user has specified `GRPC_VERBOSITY`.

Background:
gRPC Core overrides the setting of `absl::MinLogLevel` based on the setting of `GRPC_VERBOSITY`.
`GRPC_VERBOSITY` defaults to the setting of `GPR_DEFAULT_LOG_VERBOSITY_STRING` if that env var is not set, resulting in the consequence that gRPC Core would override the setting of `absl::MinLogLevel` even if the user had previously modified it.

Note that even with this change, the minimum log severity that's printed remains at `ERROR` by default.

Closes #36931

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36931 from yashykt:ChangeDefaultGrpcVerbosity ee92404c35
PiperOrigin-RevId: 644127489

src/core/lib/config/config_vars.cc

@@ -26,7 +26,7 @@
 #include "src/core/lib/config/load_config.h"
 
 #ifndef GPR_DEFAULT_LOG_VERBOSITY_STRING
-#define GPR_DEFAULT_LOG_VERBOSITY_STRING "ERROR"
+#define GPR_DEFAULT_LOG_VERBOSITY_STRING ""
 #endif  // !GPR_DEFAULT_LOG_VERBOSITY_STRING
 
 #ifdef GRPC_ENABLE_FORK_SUPPORT

src/core/lib/config/config_vars.yaml

@@ -66,7 +66,7 @@
   type: string
   prelude: |
     #ifndef GPR_DEFAULT_LOG_VERBOSITY_STRING
-    #define GPR_DEFAULT_LOG_VERBOSITY_STRING "ERROR"
+    #define GPR_DEFAULT_LOG_VERBOSITY_STRING ""
     #endif  // !GPR_DEFAULT_LOG_VERBOSITY_STRING
   default: $GPR_DEFAULT_LOG_VERBOSITY_STRING
   description:

src/core/util/log.cc

@@ -35,10 +35,6 @@
 #include "src/core/lib/gprpp/crash.h"
 #include "src/core/util/string.h"
 
-#ifndef GPR_DEFAULT_LOG_VERBOSITY_STRING
-#define GPR_DEFAULT_LOG_VERBOSITY_STRING "ERROR"
-#endif  // !GPR_DEFAULT_LOG_VERBOSITY_STRING
-
 static constexpr gpr_atm GPR_LOG_SEVERITY_UNSET = GPR_LOG_SEVERITY_ERROR + 10;
 static constexpr gpr_atm GPR_LOG_SEVERITY_NONE = GPR_LOG_SEVERITY_ERROR + 11;
 
@@ -145,6 +141,9 @@ void gpr_to_absl_verbosity_setting_init(void) {
   } else if (absl::EqualsIgnoreCase(verbosity, "NONE")) {
     absl::SetVLogLevel("*grpc*/*", -1);
     absl::SetMinLogLevel(absl::LogSeverityAtLeast::kInfinity);
+  } else if (verbosity.empty()) {
+    // Do not set absl::MinLogLevel if verbosity has not been set. Note that the
+    // default gRPC min log severity that is printed will still be ERROR.
   } else {
     LOG(ERROR) << "Unknown log verbosity: " << verbosity;
   }

tools/codegen/core/gen_config_vars.py

@@ -219,8 +219,8 @@ with open("test/core/test_util/fuzz_config_vars.h", "w") as H:
         ],
     )
 
-    print("#ifndef GRPC_TEST_CORE_UTIL_FUZZ_CONFIG_VARS_H", file=H)
-    print("#define GRPC_TEST_CORE_UTIL_FUZZ_CONFIG_VARS_H", file=H)
+    print("#ifndef GRPC_TEST_CORE_TEST_UTIL_FUZZ_CONFIG_VARS_H", file=H)
+    print("#define GRPC_TEST_CORE_TEST_UTIL_FUZZ_CONFIG_VARS_H", file=H)
     print(file=H)
     print("#include <grpc/support/port_platform.h>", file=H)
     print(file=H)
@@ -243,7 +243,7 @@ with open("test/core/test_util/fuzz_config_vars.h", "w") as H:
     print(file=H)
     print("}  // namespace grpc_core", file=H)
     print(file=H)
-    print("#endif  // GRPC_TEST_CORE_UTIL_FUZZ_CONFIG_VARS_H", file=H)
+    print("#endif  // GRPC_TEST_CORE_TEST_UTIL_FUZZ_CONFIG_VARS_H", file=H)
 
 with open("test/core/test_util/fuzz_config_vars.cc", "w") as C:
     put_copyright(C)