Prevent clippy::ignored_unit_patterns in macro expansions

This is a pedantic lint added in 1.73.0. Because it occurs inside a
macro expansion, the lint is triggered from user code. The justification
given by the lint definition is:

> Matching with `()` explicitly instead of `_` outlines the fact that
> the pattern contains no data. Also it would detect a type change that
> `_` would ignore.

Removing the lint requires a trivial change. It does introduce the
possibility for compilation errors in the event the return type of the
function currently returning `()` changes, but that seems like more of a
benefit than a drawback. In these cases, it seems unlikely that the
return type in question will change in the future.

The user experience can be seen by linting the examples:

```
% cargo +nightly clippy --examples -- -A clippy::all -W clippy::ignored_unit_patterns -Z macro-backtrace
   Compiling prometheus v0.13.3 (/home/matt/src/rust-prometheus)
...
warning: matching over `()` is more explicit
   --> /home/matt/src/rust-prometheus/src/macros.rs:217:58
    |
214 |   macro_rules! register_counter {
    |   -----------------------------
    |   |
    |   in this expansion of `register_counter!` (#1)
    |   in this expansion of `register_counter!` (#2)
    |   in this expansion of `register_counter!` (#3)
...
217 |           $crate::register(Box::new(counter.clone())).map(|_| counter)
    |                                                            ^
...
221 |           register_counter!(@of_type Counter, $OPTS)
    |           ------------------------------------------ in this macro invocation (#3)
...
225 |           register_counter!(opts!($NAME, $HELP))
    |           -------------------------------------- in this macro invocation (#2)
    |
   ::: examples/example_push.rs:16:40
    |
16  |       static ref PUSH_COUNTER: Counter = register_counter!(
    |  ________________________________________-
17  | |         "example_push_total",
18  | |         "Total number of prometheus client pushed."
19  | |     )
    | |_____- in this macro invocation (#1)
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ignored_unit_patterns
    = note: requested on the command line with `-W clippy::ignored-unit-patterns`
...
```

Signed-off-by: Matt Weber <30441572+mweber15@users.noreply.github.com>

src/macros.rs

@@ -214,7 +214,7 @@ fn test_histogram_opts_trailing_comma() {
 macro_rules! register_counter {
     (@of_type $TYPE:ident, $OPTS:expr) => {{
         let counter = $crate::$TYPE::with_opts($OPTS).unwrap();
-        $crate::register(Box::new(counter.clone())).map(|_| counter)
+        $crate::register(Box::new(counter.clone())).map(|()| counter)
     }};
 
     ($OPTS:expr $(,)?) => {{
@@ -260,7 +260,7 @@ fn test_register_counter_trailing_comma() {
 macro_rules! register_counter_with_registry {
     (@of_type $TYPE: ident, $OPTS:expr, $REGISTRY:expr) => {{
         let counter = $crate::$TYPE::with_opts($OPTS).unwrap();
-        $REGISTRY.register(Box::new(counter.clone())).map(|_| counter)
+        $REGISTRY.register(Box::new(counter.clone())).map(|()| counter)
     }};
 
     ($OPTS:expr, $REGISTRY:expr $(,)?) => {{
@@ -361,14 +361,14 @@ fn test_register_int_counter() {
 macro_rules! __register_counter_vec {
     ($TYPE:ident, $OPTS:expr, $LABELS_NAMES:expr) => {{
         let counter_vec = $crate::$TYPE::new($OPTS, $LABELS_NAMES).unwrap();
-        $crate::register(Box::new(counter_vec.clone())).map(|_| counter_vec)
+        $crate::register(Box::new(counter_vec.clone())).map(|()| counter_vec)
     }};
 
     ($TYPE:ident, $OPTS:expr, $LABELS_NAMES:expr, $REGISTRY:expr) => {{
         let counter_vec = $crate::$TYPE::new($OPTS, $LABELS_NAMES).unwrap();
         $REGISTRY
             .register(Box::new(counter_vec.clone()))
-            .map(|_| counter_vec)
+            .map(|()| counter_vec)
     }};
 }
 
@@ -543,12 +543,12 @@ fn test_register_int_counter_vec() {
 macro_rules! __register_gauge {
     ($TYPE:ident, $OPTS:expr) => {{
         let gauge = $crate::$TYPE::with_opts($OPTS).unwrap();
-        $crate::register(Box::new(gauge.clone())).map(|_| gauge)
+        $crate::register(Box::new(gauge.clone())).map(|()| gauge)
     }};
 
     ($TYPE:ident, $OPTS:expr, $REGISTRY:expr) => {{
         let gauge = $crate::$TYPE::with_opts($OPTS).unwrap();
-        $REGISTRY.register(Box::new(gauge.clone())).map(|_| gauge)
+        $REGISTRY.register(Box::new(gauge.clone())).map(|()| gauge)
     }};
 }
 
@@ -670,14 +670,14 @@ macro_rules! register_int_gauge_with_registry {
 macro_rules! __register_gauge_vec {
     ($TYPE:ident, $OPTS:expr, $LABELS_NAMES:expr $(,)?) => {{
         let gauge_vec = $crate::$TYPE::new($OPTS, $LABELS_NAMES).unwrap();
-        $crate::register(Box::new(gauge_vec.clone())).map(|_| gauge_vec)
+        $crate::register(Box::new(gauge_vec.clone())).map(|()| gauge_vec)
     }};
 
     ($TYPE:ident, $OPTS:expr, $LABELS_NAMES:expr, $REGISTRY:expr $(,)?) => {{
         let gauge_vec = $crate::$TYPE::new($OPTS, $LABELS_NAMES).unwrap();
         $REGISTRY
             .register(Box::new(gauge_vec.clone()))
-            .map(|_| gauge_vec)
+            .map(|()| gauge_vec)
     }};
 }
 
@@ -917,7 +917,7 @@ macro_rules! register_histogram {
 
     ($HOPTS:expr $(,)?) => {{
         let histogram = $crate::Histogram::with_opts($HOPTS).unwrap();
-        $crate::register(Box::new(histogram.clone())).map(|_| histogram)
+        $crate::register(Box::new(histogram.clone())).map(|()| histogram)
     }};
 }
 
@@ -974,7 +974,7 @@ macro_rules! register_histogram_with_registry {
         let histogram = $crate::Histogram::with_opts($HOPTS).unwrap();
         $REGISTRY
             .register(Box::new(histogram.clone()))
-            .map(|_| histogram)
+            .map(|()| histogram)
     }};
 }
 
@@ -1030,7 +1030,7 @@ fn test_register_histogram_with_registry_trailing_comma() {
 macro_rules! register_histogram_vec {
     ($HOPTS:expr, $LABELS_NAMES:expr $(,)?) => {{
         let histogram_vec = $crate::HistogramVec::new($HOPTS, $LABELS_NAMES).unwrap();
-        $crate::register(Box::new(histogram_vec.clone())).map(|_| histogram_vec)
+        $crate::register(Box::new(histogram_vec.clone())).map(|()| histogram_vec)
     }};
 
     ($NAME:expr, $HELP:expr, $LABELS_NAMES:expr $(,)?) => {{
@@ -1094,7 +1094,7 @@ macro_rules! register_histogram_vec_with_registry {
         let histogram_vec = $crate::HistogramVec::new($HOPTS, $LABELS_NAMES).unwrap();
         $REGISTRY
             .register(Box::new(histogram_vec.clone()))
-            .map(|_| histogram_vec)
+            .map(|()| histogram_vec)
     }};
 
     ($NAME:expr, $HELP:expr, $LABELS_NAMES:expr, $REGISTRY:expr $(,)?) => {{