Unable to push logs to Log Analytics

[koksan83]

Issue description:
Unable to push logs to Log Analytics using the provided sample config and log line. Not sure how to gather more info to troubleshoot further.

Config used:

<source>
    @type tail                         # input plugin
    path /tmp/access.log   # monitoring file
    pos_file /tmp/fluentd_pos_file     # position file
    format apache                      # format
    tag azure-loganalytics.access      # tag
</source>

<match azure-loganalytics.**>
    @type azure-loganalytics
    customer_id foobar   # Customer ID aka WorkspaceID String
    shared_key foobar     # The primary or the secondary Connected Sources client authentication key
    log_type access_FL  # The name of the event type. ex) ApacheAccessLog
    add_tag_field true
    tag_field_name access_FL_tag
</match>

Steps to reproduce the issue:

• Start docker container with ruby:2.4.1-jessie image
• Install td-agent
• Install fluent-plugin-azure-loganalytics via td-agent
• Apply above config and restart td-agent
• Append the log line from README to tailed file:

124.211.152.156 - - [10/Dec/2016:05:28:52 +0000] "GET /test/foo.html HTTP/1.1" 200 323 "-" "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.99 Safari/537.36"

What's the expected result?
Log gets sent to Azure Log Analytics

What's the actual result?
Output from /var/log/td-agent/td-agent.log:

2017-08-28 07:02:11 +0000 [fatal]: #0 Exception occured in posting to DataCollector API:
2017-08-28 07:02:11 +0000 [warn]: #0 failed to flush the buffer. retry_time=8 next_retry_seconds=2017-08-28 07:02:11 +0000 chunk="557cacc5a24ffe7d37bd7548ca8352bb" error_class=NoMethodError error="undefined method `+@' for #<String:0x007f24e4c48028>"
2017-08-28 07:02:11 +0000 [warn]: #0 suppressed same stacktrace

[yokawasa]

@koksan83
log_type value in the plugin configuration must be only 'alpha characters'.
Can you change log_type value to 'accessFL' (only alpha) instead of 'access_FL' and try again?

<match azure-loganalytics.**>
    @type azure-loganalytics
    customer_id foobar   # Customer ID aka WorkspaceID String
    shared_key foobar     # The primary or the secondary Connected Sources client authentication key
    log_type accessFL  # The name of the event type. ex) ApacheAccessLog
    add_tag_field true
    tag_field_name access_FL_tag
</match>

On my side, I'll add explanation on restriction of log_type field, also I add some code to prevent users to give non-alpha characters to log_type field.

[yokawasa]

I've release fluent-plugin-azure-loganalytics 0.3.1