Improve error strings from line protocol parser

[danielnelson]

Feature Request

When sending batches of line protocol it is very difficult to parse the error messages because the entire batch is shown.

Proposal:

Show only a section of the offending line and show the line number and column.

$ cat test
cpu,cpu=cpu0 usage_guest=0,usage_guest_nice=0,usage_idle=0,usage_iowait=0,usage_irq=0,usage_nice=0,usage_softirq=4.00,usage_steal=0,usage_system=96.00,usage_user=0 1537833474000000000 cpu,cpu=cpu0 usage_guest=0,usage_guest_nice=0,usage_idle=0,usage_iowait=0,usage_irq=0,usage_nice=invalid,usage_softirq=4.00,usage_steal=0,usage_system=96.00,usage_user=0 1537833474000000000
$ curl -v 'http://localhost:9999/write?db=telegraf' --data-binary @test

Current behavior:

Only byte offset is given

metric parse error: expected field at offset 281: "cpu,cpu=cpu0 usage_guest=0,usage_guest_nice=0,usage_idle=0,usage_iowait=0,usage_irq=0,usage_nice=0,usage_softirq=4.00,usage_steal=0,usage_system=96.00,usage_user=0 1537833474000000000\ncpu,cpu=cpu0 usage_guest=0,usage_guest_nice=0,usage_idle=0,usage_iowait=0,usage_irq=0,usage_nice=invalid,usage_softirq=4.00,usage_steal=0,usage_system=96.00,usage_user=0 1537833474000000000\n\n"

Desired behavior:

The error message contains line/column instead of offset. Column is given as a codepoint(rune) offset.

metric parse error: expected field on line: 2, column: 98: "cpu,cpu=cpu0 usage_guest=0,usage_guest_nice=0,usage_idle=0,usage_iowait=0,usage_irq=0,usage_nice=invalid,usage_softirq=4.00,usage_steal=0,usage_system=96.00,usage_user=0 1537833474000000000"

Use case:

Impossible to find cause of error with current format and common batch sizes.