refactor(telemetry): replace unused-parameter casts with maybe_unused

Mirror::store() and Mirror::changedTo() silenced an unused parameter
with a (void) cast in their telemetry-disabled arm. The warning was real:
Mirror<uint256> is instantiated in InboundTransactions.cpp. store()'s #else
arm held nothing else, so it collapses to a bare #endif; changedTo() keeps its
#else for the return.

readProcFile() and measuredTrim() are added by this PR and gain
[[nodiscard]]. MallocTrim.cpp predates the telemetry work, but neither
function exists on develop, so this annotates declarations this branch
authored rather than surrounding code. No caller discards either.

include/xrpl/telemetry/Recording.h

@@ -252,12 +252,10 @@ public:
      * @param value  The value to remember.
      */
     void
-    store(T const& value) noexcept
+    store([[maybe_unused]] T const& value) noexcept
     {
 #ifdef XRPL_ENABLE_TELEMETRY
         value_ = value;
-#else
-        (void)value;
 #endif
     }
 
@@ -270,14 +268,13 @@ public:
      *  False when compiled out, so a caller's reporting branch never runs.
      */
     [[nodiscard]] bool
-    changedTo(T const& value) noexcept
+    changedTo([[maybe_unused]] T const& value) noexcept
     {
 #ifdef XRPL_ENABLE_TELEMETRY
         bool const changed = value_ != value;
         value_ = value;
         return changed;
 #else
-        (void)value;
         return false;
 #endif
     }

src/libxrpl/basics/MallocTrim.cpp

@@ -77,7 +77,7 @@ parseStatmRSSkB(std::string const& statm)
  * @param path Absolute path of the pseudo-file.
  * @return The file contents, or an empty string if it could not be opened.
  */
-std::string
+[[nodiscard]] std::string
 readProcFile(std::string const& path)
 {
     std::ifstream ifs(path, std::ios::in | std::ios::binary);
@@ -103,7 +103,7 @@ readProcFile(std::string const& path)
  * @return A fully populated report. Fields whose source syscall failed keep
  *         their -1 "not measured" sentinel.
  */
-MallocTrimReport
+[[nodiscard]] MallocTrimReport
 measuredTrim(std::size_t padBytes)
 {
     MallocTrimReport report;