feat: add DATADOG_STORAGE_TIER env support for logs tools

[j8d]

Summary

Adds support for the DATADOG_STORAGE_TIER environment variable to enable querying logs from Datadog's Flex tier storage.

Problem

On November 17, 2025, we moved to the Flex tier. The current MCP server has no way to specify which storage tier to query, so all log searches default to the indexes tier and return no results for Flex-only logs.

Solution

• Read DATADOG_STORAGE_TIER environment variable at module initialization
• Validate against supported values: indexes, online-archives, flex
• Pass storageTier to both get_logs and get_all_services Datadog API calls when configured
• Fully backwards compatible - if env var is not set, behavior is unchanged

Usage

Set in MCP config:

{
  "mcpServers": {
    "mcp-server-datadog": {
      "env": {
        "DATADOG_STORAGE_TIER": "flex"
      }
    }
  }
}