fix: MacOS compiler errors

Summary:
An unknown change started causing compilation errors on macos. Fix these compiler errors.

Error 1: deprecated function error

```
/-------------------------------------------------------------------------------
| COMPILATION FAILED
|-------------------------------------------------------------------------------
../../src/yb/util/debug/long_operation_tracker.cc:119:22: error: no member named 'unique' in 'std::shared_ptr<yb::LongOperationTracker::TrackedOperation>'
  119 |       if (!operation.unique()) {
      |            ~~~~~~~~~ ^
../../src/yb/util/debug/long_operation_tracker.cc:124:24: error: no member named 'unique' in 'std::shared_ptr<yb::LongOperationTracker::TrackedOperation>'
  124 |         if (!operation.unique()) {
      |              ~~~~~~~~~ ^
2 errors generated.

Input files:
  src/yb/util/CMakeFiles/yb_util.dir/debug/long_operation_tracker.cc.o
  /Users/tfoucher/code/yugabyte-db3/src/yb/util/debug/long_operation_tracker.cc
Output file (from -o): src/yb/util/CMakeFiles/yb_util.dir/debug/long_operation_tracker.cc.o
\-------------------------------------------------------------------------------
```

Error2: unused variable error

```
/-------------------------------------------------------------------------------
| COMPILATION FAILED
|-------------------------------------------------------------------------------
../../src/yb/docdb/docdb_util.cc:46:19: error: unused variable 'kEmptyLogPrefix' [-Werror,-Wunused-const-variable]
   46 | const std::string kEmptyLogPrefix;
      |                   ^~~~~~~~~~~~~~~
1 error generated.

Input files:
  src/yb/docdb/CMakeFiles/yb_docdb.dir/docdb_util.cc.o
  /Users/pbalivada/code/xcode/src/yb/docdb/docdb_util.cc
Output file (from -o): src/yb/docdb/CMakeFiles/yb_docdb.dir/docdb_util.cc.o
\-------------------------------------------------------------------------------
```

Error 3: unused variable error

```
/-------------------------------------------------------------------------------
| COMPILATION FAILED
|-------------------------------------------------------------------------------
../../src/yb/util/metrics-test.cc:64:21: error: unused variable 'kTableId' [-Werror,-Wunused-const-variable]
   64 | static const string kTableId = "table_id";
      |                     ^~~~~~~~
1 error generated.

Input files:
  src/yb/util/CMakeFiles/metrics-test.dir/metrics-test.cc.o
  /Users/pbalivada/code/xcode/src/yb/util/metrics-test.cc
Output file (from -o): src/yb/util/CMakeFiles/metrics-test.dir/metrics-test.cc.o
\-------------------------------------------------------------------------------
```

Fixes #24012
Jira: DB-12899

Original commit 9feb6e8 / D38164

Test Plan:
Jenkins: urgent

On MacOS 14 arm64:
  ./yb_build.sh release --sj

Reviewers: hsunder, jason, fizaa, aagrawal

Reviewed By: jason

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D38189

src/yb/docdb/docdb_util.cc

@@ -43,12 +43,6 @@ namespace docdb {
 
 using dockv::DocPath;
 
-namespace {
-
-const std::string kEmptyLogPrefix;
-
-}
-
 Status SetValueFromQLBinaryWrapper(
     QLValuePB ql_value, const int pg_data_type,
     const std::unordered_map<uint32_t, string>& enum_oid_label_map,

src/yb/util/debug/long_operation_tracker.cc

@@ -116,12 +116,12 @@ class LongOperationTrackerHelper {
 
       TrackedOperationPtr operation = queue_.top();
       queue_.pop();
-      if (!operation.unique()) {
+      if (operation.use_count() > 1) {
         lock.unlock();
         auto stack = DumpThreadStack(operation->thread_id);
         // Make sure the task did not complete while we were dumping the stack. Else we could get
         // some other innocent stack.
-        if (!operation.unique()) {
+        if (operation.use_count() > 1) {
           LOG(WARNING) << operation->message << " running for " << MonoDelta(now - operation->start)
                        << " in thread " << operation->thread_id << ":\n"
                        << stack;

src/yb/util/metrics-test.cc

@@ -63,8 +63,6 @@ namespace yb {
 
 METRIC_DEFINE_entity(test_entity);
 
-static const string kTableId = "table_id";
-
 class MetricsTest : public YBTest {
  public:
   void SetUp() override {