Check if parameter is already declared to avoid re-declaring it.

A parameter can only be declared once. When implementing a LicecycleNode
one would normally do all the initialization in `on_configure()` and the
cleanup, deinitialization in the `on_cleanup` method. Furthermore, these
transitions could potentially be called multiple times, depending on how
the node is t ransitioned from the lifecycles are triggered. However,
this approach doesn't currently work well with Diagnostics since, in the
constructor, of `diagnostic_updater::Updater` it's declaring a static
parameter without checking whether that parameter has already been
declared. In subsequent calls to `on_configure` (and thus,
if Diagnostics are initialized there, in subsequent calls to the
`Updater` constructor, this results in the following message:

```
Original error: parameter 'diagnostic_updater.period' has already been declared
```

To avoid this, until the Foxy version, the wrapper code could explicitly
call `undeclare_parameter` on the parameter that the `Updater` is
declaring, but as of Galactic, undeclaring a static parameter is not
allowed anymore. See e.g., ros2/rclcpp#1850.

In this commit, I'm checking explicitly if the parameter has already
been registered and if so, I'm just reading its value before attempting
to re-declare it.

diagnostic_updater/include/diagnostic_updater/diagnostic_updater.hpp

@@ -392,8 +392,17 @@ class Updater : public DiagnosticTaskVector
     node_name_(base_interface->get_name()),
     warn_nohwid_done_(false)
   {
-    period = parameters_interface->declare_parameter(
-      "diagnostic_updater.period", rclcpp::ParameterValue(period)).get<double>();
+    constexpr const char* period_param_name = "diagnostic_updater.period";
+    rclcpp::ParameterValue period_param;
+    try
+    {
+      period_param = parameters_interface->declare_parameter(period_param_name, rclcpp::ParameterValue(period));
+    }
+    catch (const rclcpp::exceptions::ParameterAlreadyDeclaredException& exc)
+    {
+      period_param = parameters_interface->get_parameter(period_param_name).get_parameter_value();
+    }
+    period = period_param.get<double>();
     period_ = rclcpp::Duration::from_nanoseconds(period * 1e9);
 
     reset_timer();