Handle Azure Eventhubs returns list duplicated servers

[nqminhdl]

I'm using kafka-exporter to monitor Azure Event Hubs and connect successfully with below flags

./kafka_exporter \
    --kafka.server='<eventhub-namespace>.servicebus.windows.net:9093' \
    --sasl.enabled \
    --sasl.mechanism='plain' \
    --sasl.username='$ConnectionString' \
    --sasl.password='Endpoint=sb://<eventhub-namespace>.servicebus.windows.net/;SharedAccessKeyName=<redacted>;SharedAccessKey=<redacted>=' \
    --tls.enabled \
    --tls.insecure-skip-tls-verify \
    --log.level=debug \
    --verbosity=1 \
    --kafka.version=1.0.0

With latest release 1.7.0, I have the same issue as described in this issue #349

collected metric "kafka_consumergroup_members" { label:<name:"consumergroup" value:"delivery-*" > gauge:<value:2 > } was collected before with the same name and label values
collected metric "kafka_consumergroup_members" { label:<name:"consumergroup" value:"cashier-*" > gauge:<value:2 > } was collected before with the same name and label values
collected metric "kafka_consumergroup_members" { label:<name:"consumergroup" value:"delivery-*" > gauge:<value:2 > } was collected before with the same name and label values

This PR is to handle the case for Azure Event Hubs returns new servers with different upper case characters after RefreshMetadata.

[nqminhdl]

Hi @danielqsj,

Can you help to review this PR?
Thanks!

[nqminhdl]

Hi @danielqsj,

Can you help to review this PR?
Thanks!

[danielqsj]

LGTM，thanks @nqminhdl