Add backend expansion for Grafana $__interval and $__interval_ms macros

[CoollZzz]

This pull request adds support for Grafana's $__interval and $__interval_ms macros in IoTDB table-model SQL queries. It expands both macros using Grafana's runtime query interval and adds input validation, identifier-safe macro replacement, and unit tests for the new behavior.

Interval Macro Expansion

• Expands $__interval to an IoTDB fixed-width duration literal using ms, s, m, h, d, or w.
• Expands $__interval_ms directly to the query interval in milliseconds, following Grafana's standard macro semantics.
• Uses backend.DataQuery.Interval as the runtime interval source.

Macro Matching

• Replaces only standalone macro names.
• Handles $__interval and $__interval_ms independently when both appear in the same SQL statement.
• Preserves identifiers such as $__intervalish and $__interval_ms_extra.

Query Parameter Handling

• Adds a runtime-only IntervalMS field to the table query model.
• Populates IntervalMS from backend.DataQuery.Interval after JSON unmarshalling.
• Excludes the runtime interval field from JSON serialization and deserialization.

Error Handling

• Rejects non-positive intervals when an interval macro is used.
• Validates the interval before acquiring an RPC session.
• Retains input validation in the macro expansion helper for direct callers.
• Preserves existing behavior for queries that do not contain interval macros.

Tests

Unit tests cover:

• Expansion of $__interval and $__interval_ms
• Expansion of both macros in the same SQL statement
• Fixed-width duration formatting
• Millisecond semantics of $__interval_ms
• Zero and negative intervals
• Identifier-boundary handling
• Runtime-only query parameter behavior
• Early validation before RPC session acquisition
• Existing Grafana time macro behavior

[github-actions]

Hi, this is your first pull request in IoTDB project. Thanks for your contribution! IoTDB will be better because of you.

[JackieTien97]

There are two blocking correctness and compatibility issues in the current interval macro implementation.

[JackieTien97]

[P1] Preserve Grafana's millisecond contract for $__interval_ms. Grafana defines this macro as the query interval in milliseconds, independent of the data source's timestamp precision; the Grafana SDK likewise expands it directly from query.Interval.Milliseconds(). Scaling it here changes a 120-second interval from 120000 to 120000000/120000000000 on us/ns servers and can silently change query results. Please expand it directly with strconv.FormatInt(intervalMS, 10) and update the precision-scaling tests accordingly. If raw IoTDB timestamp units are needed, that should be a separately named, plugin-specific macro rather than changing the standard Grafana macro.

[JackieTien97]

[P1] Avoid issuing privileged metadata SQL from the normal query path. In IoTDB's table dialect, SHOW VARIABLES requires the global SYSTEM privilege, so a normal Grafana service account with only database/table read access will fail before its actual query runs. The standard $__interval_ms macro does not need server precision and this lookup should be removed. If timestamp precision is still needed for a separate raw-server-unit feature before the Go client exposes the precision already returned by the session handshake, please add an explicit validated datasource setting (ms/us/ns) as a temporary solution instead of executing SHOW VARIABLES for every query.