i18n(ja): keep gauge/count/histogram metric type values in English

[yahonda]

What is changed, added or deleted? (Required)

In the Japanese translations of the Datadog, New Relic, and Prometheus/Grafana
integration docs, the "metric type" column had translated the literal metric
type values into katakana:

• gauge -> ゲージ
• count -> カウント
• histogram -> ヒストグラム

These are literal enum values from each monitoring system's own API/metadata
(matching the English source verbatim), not prose to translate, so this PR
reverts them to English. This also makes monitor-datadog-integration.md
consistent with its sibling monitor-datadog-integration-for-tidb-x.md,
which already kept these values in English.

Files changed:

• tidb-cloud/monitor-datadog-integration.md
• tidb-cloud/monitor-new-relic-integration.md
• tidb-cloud/monitor-prometheus-and-grafana-integration.md
• tidb-cloud/prometheus-grafana-integration.md

Which TiDB version(s) do your changes apply to? (Required)

• i18n-ja-release-8.5 (TiDB Japanese documentation for TiDB 8.5 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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