[API] fix sign-conversion warnings in span & trace id (open-telemetry#3761)

Author: sjanel

api/include/opentelemetry/nostd/internal/absl/types/internal/variant.h

@@ -45,7 +45,7 @@ OTABSL_NAMESPACE_BEGIN
 template <class... Types>
 class variant;
 
-OTABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);
+OTABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, static_cast<std::size_t>(-1));
 
 template <class T>
 struct variant_size;

api/include/opentelemetry/nostd/string_view.h

@@ -124,7 +124,7 @@ class string_view
       auto found = Traits::find(data() + pos, length() - pos, ch);
       if (found)
       {
-        res = found - data();
+        res = static_cast<size_type>(found - data());
       }
     }
     return res;

api/include/opentelemetry/trace/span_id.h

@@ -17,7 +17,7 @@ class SpanId final
 {
 public:
   // The size in bytes of the SpanId.
-  static constexpr int kSize = 8;
+  static constexpr size_t kSize = 8;
 
   // An invalid SpanId (all zeros).
   SpanId() noexcept : rep_{0} {}
@@ -29,7 +29,7 @@ class SpanId final
   void ToLowerBase16(nostd::span<char, 2 * kSize> buffer) const noexcept
   {
     constexpr char kHex[] = "0123456789abcdef";
-    for (int i = 0; i < kSize; ++i)
+    for (size_t i = 0; i < kSize; ++i)
     {
       buffer[i * 2 + 0] = kHex[(rep_[i] >> 4) & 0xF];
       buffer[i * 2 + 1] = kHex[(rep_[i] >> 0) & 0xF];

api/include/opentelemetry/trace/trace_id.h

@@ -20,7 +20,7 @@ class TraceId final
 {
 public:
   // The size in bytes of the TraceId.
-  static constexpr int kSize = 16;
+  static constexpr size_t kSize = 16;
 
   // An invalid TraceId (all zeros).
   TraceId() noexcept : rep_{0} {}
@@ -35,7 +35,7 @@ class TraceId final
   void ToLowerBase16(nostd::span<char, 2 * kSize> buffer) const noexcept
   {
     constexpr char kHex[] = "0123456789abcdef";
-    for (int i = 0; i < kSize; ++i)
+    for (size_t i = 0; i < kSize; ++i)
     {
       buffer[i * 2 + 0] = kHex[(rep_[i] >> 4) & 0xF];
       buffer[i * 2 + 1] = kHex[(rep_[i] >> 0) & 0xF];