Fix prometheus metrics text exporter rendering

[adam-rl]

What this PR does / why we need it: Iterate through all text chunks that need to be emitted as part of the metric text label section before rendering the value and timestamp.

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

Before this change:

# HELP OverlayBD_Alive
# TYPE OverlayBD_Alive gauge
OverlayBD_Alive{node="1.000000 1755544318414

# HELP OverlayBD_Read_Throughtput Bytes / sec
# TYPE OverlayBD_Read_Throughtput gauge
OverlayBD_Read_Throughtput{node="pread",type="0.000000 1755544318414
OverlayBD_Read_Throughtput{node="download",type="0.000000 1755544318414

# HELP OverlayBD_QPS
# TYPE OverlayBD_QPS gauge
OverlayBD_QPS{node="pread",type="0.000000 1755544318414
OverlayBD_QPS{node="download",type="0.000000 1755544318414

# HELP OverlayBD_MaxLatency us
# TYPE OverlayBD_MaxLatency gauge
OverlayBD_MaxLatency{node="pread",type="0.000000 1755544318414
OverlayBD_MaxLatency{node="download",type="0.000000 1755544318414

# HELP OverlayBD_Count Bytes
# TYPE OverlayBD_Count gauge
OverlayBD_Count{node="pread",type="0.000000 1755544318414
OverlayBD_Count{node="download",type="0.000000 1755544318414

After this change:

# HELP OverlayBD_Alive
# TYPE OverlayBD_Alive gauge
OverlayBD_Alive{node=""}  1.000000 1755548877123

# HELP OverlayBD_Read_Throughtput Bytes / sec
# TYPE OverlayBD_Read_Throughtput gauge
OverlayBD_Read_Throughtput{node="pread",type="",mode=""}  0.000000 1755548877123
OverlayBD_Read_Throughtput{node="download",type="",mode=""}  0.000000 1755548877123

# HELP OverlayBD_QPS
# TYPE OverlayBD_QPS gauge
OverlayBD_QPS{node="pread",type="",mode=""}  0.000000 1755548877123
OverlayBD_QPS{node="download",type="",mode=""}  0.000000 1755548877123

# HELP OverlayBD_MaxLatency us
# TYPE OverlayBD_MaxLatency gauge
OverlayBD_MaxLatency{node="pread",type="",mode=""}  0.000000 1755548877123
OverlayBD_MaxLatency{node="download",type="",mode=""}  0.000000 1755548877123

# HELP OverlayBD_Count Bytes
# TYPE OverlayBD_Count gauge
OverlayBD_Count{node="pread",type=""}  0.000000 1755548877123
OverlayBD_Count{node="download",type=""}  0.000000 1755548877123