Fix multiple registration of Jaeger-related gauges

[tjquinno]

Resolves #5811

The Helidon Jaeger tracing integration registers metrics on behalf of Jaeger by acting as a Jaeger MetricsFactory. In doing so, Helidon defines the metrics metadata to be reusable so if multiple components and/or the developer's own app use Jaeger tracing we don't define duplicate metrics.

But, Helidon gauges (consistent with MicroProfile metrics) are never reusable, regardless of what the metadata says. Jaeger requests the creation of gauges, and with multiple traceable components present the second attempt to register the same gauge fails, as reported in the issue.

This fix explicitly checks for the existence of a gauge (metric name and tags) before registering it to avoid this problem.

Because of the Jaeger SPI we implement, we need to be able to find the Jaeger-typed Gauge which corresponds to each Helidon Gauge so the PR adds a map. (The way we implement gauges internally does not allow us to simply store a reference to the Jaeger gauge in our own gauge and then retrieve the Jaeger gauge from it; hence the need for the map.)

Signed-off-by: [email protected] [email protected]