monotonic sampled counter nanos should remain int64_t (#88)

These values are not expected to rollover and will not ever go as high as
the actual data values.

README.md

@@ -103,7 +103,8 @@ echo -e "t:server.requestLatency:0.042\nd:server.responseSizes:1024" | nc -u -w0
 | `X`    | Monotonic Counter with Millisecond Timestamps | The value is a monotonically increasing number, sampled at a specified number of milliseconds since the epoch. A minimum of two samples must be received in order for `spectatord` to calculate a delta value and report it to the backend. The value should be a `uint64` data type, and it will handle rollovers. <br><br> This is an experimental metric type that can be used to track monotonic sources that were sampled in the recent past, with the value normalized over the reported time period. <br><br> The timestamp in milliseconds since the epoch when the value was sampled must be included as a metric option: `X,1543160297100:monotonic.Source:42`                                                                                                                                                                    |
 
 The data type for all numbers except `C` and `X` is `double`. The `C` and `X` values are recorded as `uint64_t`, and
-the calculated deltas are passed to the backend as `double`.
+the calculated deltas are passed to the backend as `double`. Passing negative values for `uint64_t` data types will
+cause the parsed string value to rollover.
 
 ### Metric Name and Tags
 

server/spectatord.cc

@@ -575,7 +575,6 @@ auto Server::parse_line(const char* buffer) -> std::optional<std::string> {
       break;
     case 'X':
       if (extra > 0) {
-        // TODO: do we need to get this to match uint64_t?
         // extra is milliseconds since the epoch
         auto nanos = extra * 1000 * 1000;
         registry_->GetMonotonicSampled(measurement->id)

spectator/measurement.h

@@ -17,7 +17,6 @@ class Measurement {
   // Measurement(id.withTag(...), value);
   Measurement(Id&& error, double value_param) noexcept = delete;
 
-  // TODO: should this be updated to support uint64_t for `C` and `X`? I don't think so, because deltas go here.
   const Id& id;
   double value;
 

spectator/monotonic_sampled.cc

@@ -13,7 +13,7 @@ MonotonicSampled::MonotonicSampled(Id id) noexcept
       ts_{0},
       prev_ts_{0} {}
 
-void MonotonicSampled::Set(uint64_t amount, uint64_t ts_nanos) noexcept {
+void MonotonicSampled::Set(uint64_t amount, int64_t ts_nanos) noexcept {
   Update();
   absl::MutexLock lock(&mutex_);
 

spectator/monotonic_sampled.h

@@ -10,7 +10,7 @@ class MonotonicSampled : public Meter {
   explicit MonotonicSampled(Id id) noexcept;
   void Measure(Measurements* results) const noexcept;
 
-  void Set(uint64_t amount, uint64_t ts_nanos) noexcept;
+  void Set(uint64_t amount, int64_t ts_nanos) noexcept;
   auto SampledRate() const noexcept -> double;
 
  private: