dashboard: add panels for transaction operations

[DifferentialOrange]

Add panels for transaction operations (begin, commit, rollback). Metrics were introduced in 2.10.0-beta1 [1] and fixed in 2.10.4 and 2.11 [2].

1. https://github.com/tarantool/tarantool/commit/48c8dc18d6e9d7ac2f8fe2bb556acb1343deeb3c
2. https://github.com/tarantool/tarantool/commit/a5fe5c32735420da334d23d982b10e5ecbfb4f49

Closes #176

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

• [x] add Grafana panel;
• [x] run make update-tests to build new dashboard artifacts;
• [x] add example application code to generate non-null metrics;
• [x] 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;
• [x] 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.