MultiGauge.register should accept more types

[lbilger]

Describe the bug
The typical way to use a MultiGauge is converting some list to a list of Rows using a Stream. This is also done in theMultiGaugeSample example. Java's type inference is tricky here. In the example, it works because Java knows what is expected by MultiGauge.register and infers List<Row<?>>. But if you separate the conversion and assign the list to a variable, the inferred type is List<Row<Number>>. The same is true for some reason if you use the newer Stream.toList() instead of Stream.collect(Collectors.toList()).

So

            temperatures.register(
                fetchTemperatures().stream()
                                   .map(record -> Row.of(Tags.of("room", record.getRoom().name()), record.getTemperature()))
                                   .collect(Collectors.toList()));

will compile but

            temperatures.register(
                fetchTemperatures().stream()
                                   .map(record -> Row.of(Tags.of("room", record.getRoom().name()), record.getTemperature()))
                                   .toList());

and

            var rows = fetchTemperatures().stream()
                    .map(record -> Row.of(Tags.of("room", record.getRoom().name()), record.getTemperature()))
                    .collect(Collectors.toList());
            temperatures.register(rows);

will not.

I wonder if the type parameter of Row adds any value at all. But removing it would probably break existing code, so I suggest to change the signature of MultiGauge.register to accept an Iterable<? extends Row<?>> instead. Then it could also be called with a List<Row<Number>>.

I will create a PR for this small change.

Environment

• Micrometer version [e.g. 1.7.1]: 1.13.2, also on the main branch at the time of writing
• Micrometer registry [e.g. prometheus]: prometheus, but not relevant
• OS: [e.g. macOS] macOS, but not relevant
• Java version: [e.g. output of java -version]

openjdk version "21.0.2" 2024-01-16
OpenJDK Runtime Environment GraalVM CE 21.0.2+13.1 (build 21.0.2+13-jvmci-23.1-b30)
OpenJDK 64-Bit Server VM GraalVM CE 21.0.2+13.1 (build 21.0.2+13-jvmci-23.1-b30, mixed mode, sharing)

To Reproduce
See description

Expected behavior
List<Row<Number>> is accepted and no ugly casts are necessary.