tdg_dashboard: fix latency units

[DifferentialOrange]

Fix latency units for GraphQL [1], IProto [2] and REST API [3] panels.

1. https://github.com/tarantool/tdg2/blob/b283c91e17fec274d1be187fb734418a7f56a034/common/graphql.lua#L633
2. https://github.com/tarantool/tdg2/blob/7983a6a7b149ac2e9e94e052d01513076847bdff/input_processor/model_iproto.lua#L57
3. https://github.com/tarantool/tdg2/blob/98b113eb4a4f71f26a34f2522e6a93ed4e67db90/input_processor/model_rest.lua#L393

Closes #190

If this pull request introduces support for new metrics, do not forget to

• add Grafana panel;
• [x] run make update-tests to build new dashboard artifacts;
• add example application code to generate non-null metrics;
• attach a screenshot of a new panel to the PR description;
• add new Telegraf tag key, if relevant,
  • to all telegraf.conf files,
  • to "Grafana dashboard" documentation page;
• add alert example, if relevant,
  • to alerts.yml,
  • to "Alerting" documentation page;
• update supported_metrics.md.

In any case, do not forget to

• [x] link an issue;
• [x] add a CHANGELOG entry;
• update documentation guidelines;
• update README guidelines.