refactor(telemetry): use ranges algorithms in ValidationTracker, mark accessors nodiscard

Nine std::count_if calls over window1h_ and window7d_ took an iterator
pair; std::ranges::count_if takes the container. One hand-rolled
erase-while-iterating loop becomes a single std::erase_if: pending_ is a
hash_map, which is a std::unordered_map alias, so the C++20 overload applies.

The hard-trim loop below it is left as a loop on purpose. It re-tests
pending_.size() every step to stop as soon as the cap is met, which erase_if
cannot express.

The accessors return values a caller must use: 13 in ValidationTracker.h, the
two Unit.h mappers, the two HistogramBuckets.h helpers, getMeter() and
networkTypeFromId(). No caller in the chain discards any of them.

include/xrpl/beast/insight/Unit.h

@@ -53,7 +53,7 @@ enum class Unit : std::uint8_t {
  * @param unit The unit to translate.
  * @return A static, null-terminated UCUM code.
  */
-constexpr char const*
+[[nodiscard]] constexpr char const*
 otelUnitCode(Unit unit) noexcept
 {
     switch (unit)
@@ -78,7 +78,7 @@ otelUnitCode(Unit unit) noexcept
  * @param unit The unit to describe.
  * @return A static, null-terminated description.
  */
-constexpr char const*
+[[nodiscard]] constexpr char const*
 otelUnitDescription(Unit unit) noexcept
 {
     switch (unit)

include/xrpl/telemetry/HistogramBuckets.h

@@ -144,7 +144,7 @@ inline constexpr std::array kByteBuckets{
  * @return true when the ladder is non-empty, starts at or above zero, and
  *         every later edge is strictly greater than its predecessor.
  */
-constexpr bool
+[[nodiscard]] constexpr bool
 isAscendingNonNegative(std::span<double const> ladder) noexcept
 {
     if (ladder.empty() || ladder.front() < 0.0)
@@ -167,7 +167,7 @@ static_assert(isAscendingNonNegative(kByteBuckets));
  * @param ladder Bucket upper bounds.
  * @return A vector holding the same edges in the same order.
  */
-inline std::vector<double>
+[[nodiscard]] inline std::vector<double>
 toVector(std::span<double const> ladder)
 {
     return std::vector<double>(ladder.begin(), ladder.end());

include/xrpl/telemetry/Telemetry.h

@@ -393,7 +393,7 @@ public:
      * @param name  Meter name used to identify the instrumentation scope.
      * @return A shared pointer to the Meter.
      */
-    virtual opentelemetry::nostd::shared_ptr<opentelemetry::metrics::Meter>
+    [[nodiscard]] virtual opentelemetry::nostd::shared_ptr<opentelemetry::metrics::Meter>
     getMeter(std::string_view name = kMeterName) = 0;
 
     /**
@@ -486,7 +486,7 @@ makeTelemetrySetup(
  * @param networkId  The network identifier from [network_id] config.
  * @return "mainnet" (0), "testnet" (1), "devnet" (2), or "unknown".
  */
-std::string
+[[nodiscard]] std::string
 networkTypeFromId(std::uint32_t networkId);
 
 }  // namespace xrpl::telemetry