Have metric-schema accept numeric Gauges only

[henryptung]

Gauges only work properly with Prometheus if they are numeric-typed, i.e. convertible to float64. To enforce this in Java, replace Gauge<?> with Gauge<? extends Number>.

Before this PR

Gauge declarations are untyped, allowing for any value type even though only numbers are legal.

After this PR

==COMMIT_MSG==
Gauges will only accept numeric types, matching the constraints of Prometheus (thus ensuring the Gauge
won't be silently dropped downstream).
==COMMIT_MSG==

Possible downsides?

Library consumers may need to fix bugs where gauges are returning
non-numeric types.

[changelog-app]

Generate changelog in changelog/@unreleased

What do the change types mean?

• feature: A new feature of the service.
• improvement: An incremental improvement in the functionality or operation of the service.
• fix: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way.
• break: Has the potential to break consumers of this service's API, inclusive of both Palantir services
  and external consumers of the service's API (e.g. customer-written software or integrations).
• deprecation: Advertises the intention to remove service functionality without any change to the
  operation of the service itself.
• manualTask: Requires the possibility of manual intervention (running a script, eyeballing configuration,
  performing database surgery, ...) at the time of upgrade for it to succeed.
• migration: A fully automatic upgrade migration task with no engineer input required.

Note: only one type should be chosen.

How are new versions calculated?

• ❗The break and manual task changelog types will result in a major release!
• 🐛 The fix changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease.
• ✨ All others will result in a minor version release.

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Gauges now require numeric return types. This matches the constraints of Prometheus
and avoids having non-numeric values silently dropped downstream.

Check the box to generate changelog(s)

• Generate changelog entry

[svc-autorelease]

Released 0.23.0