chore: fix sign-conversion warnings in span & trace id

Author: sjanel

api/include/opentelemetry/trace/span_id.h

@@ -31,8 +31,8 @@ class SpanId final
     constexpr char kHex[] = "0123456789abcdef";
     for (int i = 0; i < kSize; ++i)
     {
-      buffer[i * 2 + 0] = kHex[(rep_[i] >> 4) & 0xF];
-      buffer[i * 2 + 1] = kHex[(rep_[i] >> 0) & 0xF];
+      buffer[static_cast<size_t>(i) * 2U + 0] = kHex[(rep_[i] >> 4) & 0xF];
+      buffer[static_cast<size_t>(i) * 2U + 1] = kHex[(rep_[i] >> 0) & 0xF];
     }
   }
 

api/include/opentelemetry/trace/trace_id.h

@@ -37,8 +37,8 @@ class TraceId final
     constexpr char kHex[] = "0123456789abcdef";
     for (int i = 0; i < kSize; ++i)
     {
-      buffer[i * 2 + 0] = kHex[(rep_[i] >> 4) & 0xF];
-      buffer[i * 2 + 1] = kHex[(rep_[i] >> 0) & 0xF];
+      buffer[static_cast<size_t>(i) * 2U + 0] = kHex[(rep_[i] >> 4) & 0xF];
+      buffer[static_cast<size_t>(i) * 2U + 1] = kHex[(rep_[i] >> 0) & 0xF];
     }
   }