pw_metric: Make constructors constexpr

This CL makes the following constructors constexpr:

- pw::metric::Group(Token)
- pw::metric::Metric(Token, float)
- pw::metric::Metric(Token, uint32_t)
- pw::metric::TypedMetric<float>(Token, float)
- pw::metric::TypedMetric<uint32_t>(Token, uint32_t)

Change-Id: I601d76cda0f5a03747df8282337da7afa94fe3db
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/172379
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Aaron Green <aarongreen@google.com>

pw_metric/metric.cc

@@ -109,8 +109,6 @@ void Metric::Dump(IntrusiveList<Metric>& metrics, int level) {
   }
 }
 
-Group::Group(Token name) : name_(name) {}
-
 Group::Group(Token name, IntrusiveList<Group>& groups) : name_(name) {
   groups.push_front(*this);
 }

pw_metric/public/pw_metric/metric.h

@@ -65,10 +65,10 @@ class Metric : public IntrusiveList<Metric>::Item {
   void operator=(const Metric&) = delete;
 
  protected:
-  Metric(Token name, float value)
+  constexpr Metric(Token name, float value)
       : name_and_type_((name & kTokenMask) | kTypeFloat), float_(value) {}
 
-  Metric(Token name, uint32_t value)
+  constexpr Metric(Token name, uint32_t value)
       : name_and_type_((name & kTokenMask) | kTypeInt), uint_(value) {}
 
   Metric(Token name, float value, IntrusiveList<Metric>& metrics);
@@ -112,7 +112,7 @@ class TypedMetric;
 template <>
 class TypedMetric<float> : public Metric {
  public:
-  TypedMetric(Token name, float value) : Metric(name, value) {}
+  constexpr TypedMetric(Token name, float value) : Metric(name, value) {}
   TypedMetric(Token name, float value, IntrusiveList<Metric>& metrics)
       : Metric(name, value, metrics) {}
 
@@ -129,7 +129,7 @@ class TypedMetric<float> : public Metric {
 template <>
 class TypedMetric<uint32_t> : public Metric {
  public:
-  TypedMetric(Token name, uint32_t value) : Metric(name, value) {}
+  constexpr TypedMetric(Token name, uint32_t value) : Metric(name, value) {}
   TypedMetric(Token name, uint32_t value, IntrusiveList<Metric>& metrics)
       : Metric(name, value, metrics) {}
 
@@ -148,7 +148,7 @@ class TypedMetric<uint32_t> : public Metric {
 // Size: 16 bytes/128 bits - next, name, metrics, children.
 class Group : public IntrusiveList<Group>::Item {
  public:
-  Group(Token name);
+  constexpr Group(Token name) : name_(name) {}
   Group(Token name, IntrusiveList<Group>& groups);
 
   Token name() const { return name_; }