Javadoc reactor fails on WebServer metrics module-link mismatch

## Problem

The Javadoc build fails in `helidon-webserver-observe-metrics` when
`helidon-metrics-providers-micrometer` is also part of the active reactor.

The Javadoc tool reports:

```text
warning: The code being documented uses packages in the unnamed module, but the packages defined in .../helidon-metrics-providers-micrometer/apidocs/ are in named modules.
```

Because the WebServer project sets `javadoc.fail-on-warnings` to `true`, this
warning fails the build.

## Reproducer

Using JDK 26 on `main`:

```shell
mvn clean package -Pjavadoc -DskipTests \
    -pl :helidon-metrics-providers-micrometer,:helidon-webserver-observe-metrics
```

Restarting from `:helidon-webserver-observe-metrics` succeeds because the
Micrometer provider is no longer part of the active reactor. The two-module
reproducer also succeeds with `-DdetectOfflineLinks=false`, confirming that the
failure comes from the automatically detected reactor Javadoc link.

## Cause

`webserver/observe/metrics/pom.xml` declares
`helidon-metrics-providers-micrometer` as a compile dependency, although the
WebServer metrics module does not use provider types and its module descriptor
does not require the provider module. Maven therefore documents the dependency
as part of the unnamed module while linking to Javadocs whose element list
declares the named `io.helidon.metrics.providers.micrometer` module.

## Proposed solution

Declare `helidon-metrics-providers-micrometer` with `runtime` scope in
`webserver/observe/metrics/pom.xml`. The provider remains available at runtime
but is removed from the compile and Javadoc class paths, avoiding the modularity
mismatch.

The two-module reproducer and the full reactor Javadoc build should then pass
without disabling offline links or relaxing `javadoc.fail-on-warnings`.

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