metrics endpoint is broken with signalfx-metadata plugin

[karthikic]

curl -g collectd-exporter.monitoring.svc:9103/metrics 
An error has occurred during metrics collection:

collected metric collectd_signalfx_metadata_gauge gauge:<value:910.000819206238 >  was collected before with the same name and label values```

This issue comes with latest version (0.4)

when i downgrade it to (0.3.1) , i see invalid Prometheus metric name being generated

curl -g collectd-exporter.monitoring.svc:9103/metrics | grep "collectd"
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  8122  100  8122   # HELP collectd_exporter_build_info A metric with a constant '1' value labeled by version, revision, branch, and goversion from which collectd_exporter was built.
 # TYPE collectd_exporter_build_info gauge
0     collectd_exporter_build_info{branch="cut-0.3.0",goversion="go1.6.2",revision="3abb95c",version="0.3.1"} 1
0  # HELP collectd_last_push_timestamp_seconds Unix timestamp of the last received collectd metrics push in seconds.
127# TYPE collectd_last_push_timestamp_seconds gauge
3kcollectd_last_push_timestamp_seconds 1.5447246335487258e+09
    # HELP collectd_signalfx-metadata_counter_total Collectd exporter: 'signalfx-metadata' Type: 'counter' Dstype: 'api.Counter' Dsname: 'value'
  # TYPE collectd_signalfx-metadata_counter_total counter
0 collectd_signalfx-metadata_counter_total 0
--:# HELP collectd_signalfx-metadata_gauge Collectd exporter: 'signalfx-metadata' Type: 'gauge' Dstype: 'api.Gauge' Dsname: 'value'
--# TYPE collectd_signalfx-metadata_gauge gauge
:collectd_signalfx-metadata_gauge 13252.9735565186
--collectd_signalfx-metadata_gauge 1430.0007982254
 -# HELP collectd_statsd_count Collectd exporter: 'statsd' Type: 'count' Dstype: 'api.Gauge' Dsname: 'value'
# TYPE collectd_statsd_count gauge
-:-collectd_statsd_count{instance="collectd-statsd-7f7587bc46-qj2nh",statsd="prometheus_target_kbasync_lenh_secc"} 100
-:--# HELP collectd_statsd_derive Collectd exporter: 'statsd' Type: 'derive' Dstype: 'api.Derive' Dsname: 'value'
 -# TYPE collectd_statsd_derive counter
collectd_statsd_derive{instance="collectd-statsd-7f7587bc46-qj2nh",statsd="prometheus_target_kbasync_lenh_secc"} 300
-:--:-- 1321k

here invalid metric names are collectd_signalfx-metadata_counter_total notice the -

In both cases it causes a scraping error in prometheus.