This was primarily intended to consume InfluxDB-style expvars,
particularly InfluxDB's `/debug/vars` endpoint.
That endpoint follows a structure like
```json
{
"httpd::8086": {
"name": "httpd",
"tags": {
"bind": ":8086"
},
"values": {
"pointsWrittenOK": 33756,
"queryReq": 19,
"queryRespBytes": 26973,
"req": 428,
"writeReq": 205,
"writeReqBytes": 3939161
}
}
}
```
There are an arbitrary number of top-level keys in the JSON response at
the configured URLs, and this plugin will iterate through all of their
values looking for objects with keys "name", "tags", and "values"
indicating a metric to be consumed by telegraf.
Running this on current master of InfluxDB, I am able to record nearly
the same information that is normally stored in the `_internal`
database; the only measurement missing from `_internal` is `runtime`,
which is present in expvars under the "memstats" key but does not follow
the expvar format and so is not consumed in this plugin.
```
$ influx -database=telegraf -execute 'SHOW FIELD KEYS FROM /expvar/'
name: expvar_influxdb_engine
----------------------------
fieldKey
blksWrite
blksWriteBytes
blksWriteBytesC
pointsWrite
pointsWriteDedupe
name: expvar_influxdb_httpd
---------------------------
fieldKey
pingReq
pointsWrittenOK
queryReq
queryRespBytes
req
writeReq
writeReqBytes
name: expvar_influxdb_shard
---------------------------
fieldKey
fieldsCreate
seriesCreate
writePointsOk
writeReq
name: expvar_influxdb_subscriber
--------------------------------
fieldKey
pointsWritten
name: expvar_influxdb_wal
-------------------------
fieldKey
autoFlush
flushDuration
idleFlush
memSize
metaFlush
pointsFlush
pointsWrite
pointsWriteReq
seriesFlush
name: expvar_influxdb_write
---------------------------
fieldKey
pointReq
pointReqLocal
req
subWriteOk
writeOk
```
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