[v10.2.x] InfluxDB: Fix parsing multiple tags on backend mode

[itsmylife]

Backport 6b13064 from #77340

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What is this feature?

This is fixing the response when we have multiple tags in the response. Multiple tags must be separated by a comma in the result. This also allows us to be in sync with the frontend mode result.

Why do we need this feature?

To have a proper output

Who is this feature for?

InfluxDB InfluxQL backend mode users

Special notes for your reviewer:

How to test?

• Run dev environment with feature flag influxdbBackendMigration=true
• Use this query as an example SELECT "usage_idle" FROM "cpu" WHERE $timeFilter GROUP BY "cpu"::tag, "host"::tag
• Observe that you get the response (see it on the legend) back as cpu.usage_idle {cpu: cpu3 host: 41c9dcf3b960}
• Switch to this branch and run the query again
• Observe that you get the response (see it on the legend) back as cpu.usage_idle {cpu: cpu3, host: 41c9dcf3b960}