Update module github.com/prometheus/client_golang to v0.12.1

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
github.com/prometheus/client_golang	v0.9.2 -> v0.12.1

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Release Notes

prometheus/client_golang (github.com/prometheus/client_golang)

v0.9.4: 0.9.4 / 2019-06-07

Compare Source

• [CHANGE] API client: Switch to alert values as strings. #​585
• [FEATURE] Add a collector for Go module build information. #​595
• [FEATURE] promhttp: Add an counter for internal errors during HTTP exposition. #​594
• [FEATURE] API client: Support target metadata API. #​590
• [FEATURE] API client: Support storage warnings. #​562
• [ENHANCEMENT] API client: Improve performance handling JSON. #​570
• [BUGFIX] Reduce test flakiness. #​573

v0.9.3: 0.9.3 / 2019-05-16

Compare Source

• [CHANGE] Required Go version is now 1.9+. #​561
• [FEATURE] API client: Add POST with get fallback for Query/QueryRange. #​557
• [FEATURE] API client: Add alerts endpoint. #​552
• [FEATURE] API client: Add rules endpoint. #​508
• [FEATURE] push: Add option to pick metrics format. #​540
• [ENHANCEMENT] Limit time the Go collector may take to collect memstats,
  returning results from the previous collection in case of a timeout. #​568
• [ENHANCEMENT] Pusher now requires only a thin interface instead of a full
  http.Client, facilitating mocking and custom HTTP client implementation.
  #​559
• [ENHANCEMENT] Memory usage improvement for histograms and summaries without
  objectives. #​536
• [ENHANCEMENT] Summaries without objectives are now lock-free. #​521
• [BUGFIX] promhttp: InstrumentRoundTripperTrace now takes into account a pre-set context. #​582
• [BUGFIX] TestCounterAddLarge now works on all platforms. #​567
• [BUGFIX] Fix promhttp examples. #​535 #​544
• [BUGFIX] API client: Wait for done before writing to shared response
  body. #​532
• [BUGFIX] API client: Deal with discovered labels properly. #​529

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