add STRING number support

[kmuto]

I found mackerel-plugin-snmp failed to parse a number of STRING type from SNMP device, like strconv.ParseFloat: parsing "[51 46 49 52]": invalid syntax.
For example, original value from SNMP device is '3.14', and 51 46 49 52 seems the byte array of '3.14'.
Because it is usually impossible to change a type on the SNMP device side, the receiver has a responsiblity to accept it.

Here is a patch to support STRING number.

• parseFloat again for byte array with forcing String conversion by fmt.Sprintf('%s' only if the parsing failed first. resp.Variables[0].Value may be vary, so original first fmt.Sprint(resp.Variables[0].Value) works well, except a byte array of string.
• To test it, extend command is added. It will back STRING 3.14. .1.3.6.1.4.1.8072.1.3.2.3.1.2.4.101.99.104.111 = STRING: 3.14

Result of test.sh
Before:

2024/03/29 08:47:18 strconv.ParseFloat: parsing "[51 46 49 52]": invalid syntax
graphite-metric-test: <stdin>: rule snmp.echo[>0] is not matched any metrics
test-mackerel-plugin-snmp
test-mackerel-plugin-snmp

After:

test-mackerel-plugin-snmp
test-mackerel-plugin-snmp