What this PR does / why we need it:
This PR defaults Loki mixin dashboards to use TSDB, which is now the preferred index format. It adds a configuration variable to easily toggle between TSDB and BoltDB dashboards, and includes TSDB shipper monitoring metrics that were previously missing.

Which issue(s) this PR fixes:
Fixes #9772

Special notes for your reviewer:

• The tsdb config variable in config.libsonnet defaults to true, making TSDB the default
• When tsdb: true, BoltDB Shipper panels are hidden and TSDB Shipper panels are shown
• Added new metrics loki_tsdb_shipper_request_duration_seconds for monitoring TSDB shipper performance
• The implementation follows the existing pattern used for SSD mode configuration

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

Tested
Tested the dashboard toggle functionality based on tsdb configuration!

After running make loki-mixin, tested in local Grafana:

• tsdb: true (default) → Only TSDB Shipper section is displayed

• tsdb: false → Only BoltDB Shipper section is displayed